database: reorder where conditions so that existing compound index is used, add user_id index

Created by: sashaostrikov

First I noticed that this query will not use the index. Reordering the predicates will fix it.

Then I recognised that another query will also full scan the table because user_id index was deleted some time ago, but it should not be deleted as existing compound index "sub_repo_permissions_repo_id_user_id_version_uindex" UNIQUE, btree (repo_id, user_id, version) is inefficient for user_id or version queries. That's why I returned the user_id column index.