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: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Closes #14694.

* I got an error when updating GDK because it already exists.

According to endbosses' suggestions.

On PostgreSQL the index is created concurrently.

Fixes gitlab-org/gitlab-ce#14325

The user has the rights of a public user execpt it can never create a project,
 group, or team. Also it cant view internal projects.

It's unclear how these blank values got added, but GitLab.com had a few:

```
irb(main):002:0> Note.where("line_code IS NOT NULL AND line_code = ''").count
=> 439
```

We've added a migration to convert any existing records to use a NULL
value when blank, and updated Note to set blank values to nil before
validation.

This explains the user what they need to run and where to go in case
they want to learn more about "CREATE EXTENSION".

This also includes e.g. the appearances table which apparently wasn't
already included in the schema.

This allows the LIKE condition to use an index. Without a GIN + trigram
index LIKE queries using a wildcard at the start _won't_ use an index
and instead perform a sequence scan.

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

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

Prevent Groups to have smaller visibility than projects
Add default_group_visibility_level to configuration
Code improvements

This wasn't included in gitlab-org/gitlab-ce!3000.

This was removed from the interface in
https://github.com/gitlabhq/gitlabhq/pull/6027 but its implementation
lingered around for two years.

Closes #1980