This chance is broken up in two steps:

1. Use ILIKE on PostgreSQL and LIKE on MySQL, instead of using
   "WHERE lower(x) LIKE lower(y)" as ILIKE is significantly faster than
   using lower(). In many cases the use of lower() will force a slow
   sequence scan.

2. Instead of using 1 query that searches both projects and namespaces
   using a JOIN we're using 2 separate queries that are UNION'd
   together. Using a JOIN would force a slow sequence scan, using a
   UNION avoids this.

This method now uses Arel as Arel automatically uses ILIKE on PostgreSQL
and LIKE on MySQL, removing the need to handle this manually.