Adds service desk verification triggered email

Feature context

Click to expand 👇

Right now it is not possible to customize the Service Desk email address (intake and sending) in its entirety. On self-hosted instances you have more control over the used addresses, but you will still have a rather cryptic target email address for a specific service desk in a project. For .com users it's currently not possible to customize the Service Desk email at all.

There is a proposal and a further exploration around this issue. A summary of the solution path is the following: Users set up their custom email to forward all emails to the cryptic Service Desk email and provide SMTP credentials so we can send emails on their behalf. This way customers seeking support will only see the custom email address in their communication.

There is further discussion about improving and changing the general infrastructure, but this approach is a MVC to solve the issue for our customers.

🗺 How does it contribute to the whole feature?

Click to expand 👇

This MR is the second part in a series of MRs that will follow in order to complete this feature. See #329990 (comment 1227384943) for a detailed breakdown. Here's a summary:

1. ✅ Using SMTP credentials. Foundation work. Add Service Desk custom email foundation (!108017 - merged)
2. 🎯 Verify email ownership, correct function and setup Part 2: Move credentials to own table to address additional access methods
3. Ingest replies from custom email
4. Add settings and validation to Settings page
5. Add documentation

What does this MR do and why?

🎏 The feature as a whole is hidden behind the feature flag service_desk_custom_email and is not enabled on production yet

This MR adds one of three emails we need for Service Desk custom email address setup end verification. This email will be sent out, once credentials were added and the verification process was triggered. The user who triggered the verification and all project owners will receive this notification message (logic will be delivered in an upcoming MR via a service).

The purpose of the email is to raise awareness that someone added a custom email address and initiated a verification process.

This MR is also a dependency MR for other upcoming custom email related email MRs. I will base those MRs on top of this one and add it as a dependency.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Email: service_desk_verification_triggered_email

HTML	Text

How to set up and validate locally

Easy path (rails mailer previews)

1. Open http://127.0.0.1:3000/rails/mailers/notify/service_desk_verification_triggered_email and view the HTML and text version of this mail

Letter opener path (send via console)

1. Find a project ID in your installation that you have not used for any Service Desk (including CustomEmailCredentials and CustomEmailVerification testing) setup and testing. Why? It makes these steps a lot easier 🙂
2. Open the rails console bin/rails c in gitlab folder
3. Find the project by id

   project = Project.find(5) # Where 5 is your project id

4. Create ServiceDeskSetting entry (which holds Service Desk related meta info and custom email stuff)

   settings = ServiceDeskSetting.create!(
     project: project,
     custom_email: 'user@gmail.com'
   )

5. Create ServiceDesk::CustomEmailCredential

   credential = ServiceDesk::CustomEmailCredential.create!(
      project: project,
      smtp_address: 'smtp.gmail.com', # Use gmail, because Gitlab::UrlBlocker resolves DNS
      smtp_port: 587,
      smtp_username: 'user@gmail.com',
      smtp_password: 'supersecret'
    )

6. Create ServiceDesk::CustomEmailVerification entry (which holds meta info for the state of the verification process)

   verification = ServiceDesk::CustomEmailVerification.create!(
      project: project,
      token: 'XXXXXXXXXXXX',
      triggerer: User.first, # that should be @root, that's fine
      triggered_at: Time.current,
      state: 0 # which is "running"
    )

7. Reset associations on project, because we access these objects from ServiceDeskSetting internally via project

   project.reset

8. Actually send the mail

   Notify.service_desk_verification_triggered_email(settings, 'owner@example.com').deliver

9. Open Letter Opener at http://127.0.0.1:3000/rails/letter_opener/. You should see a new email that states that @root updates the custom email address settings for your project and provided credentials for user@gmail.com.
10. (Optional) clean up the mess and remove entries

    settings.destroy
    credential.destroy
    verification.destroy

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.