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.

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

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