Rework project authorizations and nested groups for better performance

This MR reworks the queries used for calculating project authorizations, in particular those used for getting nested groups.

TODO

• Re-use code in Users::RefreshAuthorizedProjectsService between MySQL and PostgreSQL (as much as possible)
• Add Group.supports_nested_groups? which returns true for PostgreSQL, false otherwise
• In the UI use Group.supports_nested_groups? to hide nested groups related UI elements
• Do something similar in controllers or models (depending on what the best place would be)
• Add a migration which converts nested groups to regular groups for MySQL, making sure the right people still have access
• Remove hierarchy related methods that use the old routes setup and are no longer necessary
• Add documentation stating nested groups can not be supported properly on MySQL

Performance Impact

All tests were performed using the GitLab.com database, on a read-only replica.

Method	Before	After	Notes
Namespace#descendants	580 ms	~1 ms	Tests performed using the "gitlab-org" group
Namespace#ancestors	~1 ms	~1 ms	Tests performed using the "sub-group-test" group. Old query was a regular index scan

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together