Notifications for when pipelines are fixed
### Problem to solve
If a pipeline becomes red/failed, an email is sent as notification. However, if that pipeline becomes green/successful again, no update is sent. In order to know, a person would have to proactively go into the web experience to check. This is not an effective use of their time - if we think it's important enough to send an email when they break, it is important enough to send an email when restored.
### Proposal
- Send an e-mail for the **first** green/successful pipeline after a red/failed pipeline on the same commit, branch, or tag.
### Testing required before rollout?
https://gitlab.com/gitlab-org/gitlab/-/issues/24309#note_348921044 details the decision that we expect no user frustration with 2-3 emails per merge request more after this change. Additionally, there will be a dedicated blog post created for this change informing users.
## Sample Email

## Troubleshooting
### You want to stop receiving `Your pipeline has been fixed!` notification (a.k.a. fixed pipeline notification)
This feature is currently enabled on gitlab.com and the notification setting for fixed emails is **enabled by default**.
#### Disable it for a particular project
If you'd like to disable the setting for a particular project only, please follow these steps:
- Visit a project
- Click the bell mark icon and click **Custom**.
- Check off the **Fixed pipeline** checkbox

#### Disable it for all of your projects
To disable the notification for all projects, please follow these steps:
- Click your user icon at right top and click **Profile**
- Click **Edit Profile** button at right top.
- Click **Notifications >> Global notification level >> Custom**
- Check off the **Fixed pipeline** checkbox

_NOTE: When you change the notification level to Custom, you might accidentally enable the other notification checkbox. Please make sure that all of the notification checkboxes are correctly checked._
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