project_authorization.rb

Prior to this commit the refreshing of authorized projects was done in
two steps:

1. Remove existing authorizations
2. Insert a new list of all authorizations

This can lead to a high amount of dead tuples as every time all rows are
being replaced. For example, if a user with 100 authorizations is given
access to a new project this would lead to:

* 100 rows being removed
* 101 new rows being inserted

This commit changes the way this system works so it only removes/inserts
what is necessary. Using the above example this would lead to only 1 new
row being inserted, with the initial 100 being left untouched.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/25257