Display MR unmergeable reasons

Closes #23460

Adds `#build_notification_recipients` to `NotificationRecipientService`
that returns the `NotificationRecipient` objects in order to be able to
access the new attribute `reason`.

This new attribute is used in the different notifier methods in order to
add the reason as a header: `X-GitLab-NotificationReason`.

Only the reason with the most priority gets sent.

Notification settings are described, but aren't mentioned where they
are or where to change them.

1. Never send a pipeline email to anyone other than the user who created
   the pipeline.
2. Only send pipeline success emails to people with the custom
   notification setting for enabled. Watchers and participants will
   never receive this.
3. When custom settings are unset (for new settings and legacy ones),
   act as if failed_pipeline is set.

Closes #24845

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6342#note_17981421

This slightly changes the semantics of the 'New Issue' and 'New MR' events to
include new mentions in edited Mentionables.

An alternative would be to introduce 'Issue updated' and 'MR updated' events,
but that would lead to questions about why those events were only available
to new mentions, and not existing mentions as well, so hold off for now.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

product names, plus a couple of instances of bad Markdown markup.