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.

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>

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.

The only major difference with the EE version is the change from a light and dark logo to only a header logo
The dark logo wasn't used anyway, so it seemed to make sense to me to rename the field to the actual function of it