Optimize /admin/applications so that it does not timeout

On our dev instance, /admin/applications as not loading because:

1. There was an unindexed query by application_id.
2. There was an expensive query that attempted to load 1 million unique entries via ActiveRecord just to find the unique count.

We fix the first issue by adding an index for that column.

We fix the second issue with a simple SELECT COUNT(DISTINCT resource_owner_id) SQL query. There was also an N+1 query that was fixed as well.

In addition, we add pagination to avoid loading more than 20 applications at once.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/67228