Run 'git gc' every 10 pushes

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/14250

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/12620

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/254

Associated gitlab-shell MR:
https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/44

See merge request !3206

Bring from EE: Share Project with Group

- [x] Models and migrations
- [x] Logic, UI
- [x] Tests
- [x] Documentation
- [x] Share with group lock
- [x] Api feature
- [x] Api docs
- [x] Api tests

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

For #12831 

cc @DouweM @rspeicher @vsizov

See merge request !3186

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

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