Partially addresses #47424.

Closes #23460

This was migrated to separate columns in 9.4, and now just needs to be removed
for real.

Create a post-deployment migration to update all existing notification settings
with at least one custom level enabled to the new format. Also handle the same
conversion when updating settings, to catch any stragglers.

Add columns for each custom notification level, defaulting to null. Read from
those columns if non-null, otherwise fall back to the serialized column. Writing
will write to the new column if `events` isn't manually set.

One should really use a separate table instead of using polymorphic
associations.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11168 for
more information.

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.

for watchers. Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8176#note_20920905

This reverts commit 1404aa86.

If the Sidekiq job fails for some reason, a project can be 'stuck'
pending deletion. The project can't be viewed, so it shouldn't be
available through the notification settings association as this will
throw an exception when we try to show the link.

This reverts commit 13e37a3e.

This is part of #12758

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

In 8278b763 the default behaviour of annotation
has changes, which was causing a lot of noise in diffs. We decided in #17382
that it is better to get rid of the whole annotate gem, and instead let people
look at schema.rb for the columns in a table.

Fixes: #17382

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

* visiting project will create notification setting if missing
* change notification setting per project even without membership
* use notification settings instead of membership on profile page

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

It will hold notification setting per group or per project. It will
allow get rid of notification level stored in Member model

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>