Notification email for newly authorized and created OAuth applications

What does this MR do and why?

To try to close #414375 & send email when a user create a new application.

This MR does two things:

1. send an notification email when a user create a new application. This application can be with three level: User owned application, Group owned application, or Instance-wide application
2. send an notification email to the user when they authorized an existing application. This is issue #414375

How to set up and validate locally

Review mail through mail preivew [1]

review notification email when a user create a new application

http://127.0.0.1:3000/rails/mailers/notify/application_created_email

review notification email when a user authorized an existing application

http://127.0.0.1:3000/rails/mailers/notify/application_authorized_email

[1]. https://docs.gitlab.com/ee/development/emails.html#mailer-previews

Test when a new application is created

create a User owned application

1. create a new application.
2. go to http://localhost:3000/rails/letter_opener/
3. there should be a new email!

create a group owned application

1. create a new group, and create a new group application at http://127.0.0.1:3000/groups/<new group>/-/settings/applications
2. go to http://localhost:3000/rails/letter_opener/
3. there should be a new email

create an Instance-wide application

1. go to admin page, and find admin application settings at here: http://127.0.0.1:3000/admin/applications.
2. create a new instance application
3. go to http://localhost:3000/rails/letter_opener/
4. there should be a new email.

Test when a user authorized an existing application

I am doing this in a complicated way.. not sure if there is some easier way to check this. The original issue description mentioned that we can use glab auth login to test but I found that glab auth login might only be used on gitlab.com as its client id is hardcoded [1]. There is no way for me to change the client id in the instance application.

An issue has been raised to address this, by the time when I was testing, the issue has not been resolved so I am using gitpod to test this instead.

The method below needs a gitpod account.

1. start the instance on gitpod so that there will be an public host name of a test GitLab instance.
2. create a new admin application when using root account.
3. find an external service provide (I use gitpod.io). Then add the gitpod instance as the new git interations [2].
4. finishing authentication, then go to https://gitpod.gitlab.instance/rails/letter_opener. There should be a letter saying an application is authorized by the root.
5. In the Git provider section at gitpod integration page, there would be a new provider, and the username is root. Delete that provider.
6. in GitLab instance, logout root user, and login as a normal user, then go to any project of this user. Add https://gitpod.io# in front of the project url.
7. In the context URL, it should ask to authorize. click okay.
8. Go to check https://gitpod.gitlab.instance/-/profile/applications for the normal, there should be a new authorized application.
9. Go to check https://gitpod.gitlab.instance/rails/letter_opener/. Now there should be a new letter for this normal user. The letter is like below.

[1]. https://gitlab.com/gitlab-org/cli/-/blob/main/pkg/oauth2/oauth2.go#L23

[2]. https://gitpod.io/user/integrations

MR acceptance checklist

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