frontend: Only do github.com prefix search optimization on dot-com

Created by: keegancsmith

We have a heuristic which translates repo queries with the prefix github.com into ^github.com. This was likely added with the sourcegraph.com use case in mind (3.5mil repos in the table and they all start with github.com). However, for customers github.com can appear in other parts of the repo string. As such we only use this optimization on Sourcegraph.com

This commit additionally includes some extra checks to ensure our include pattern parsing doesn't make the same mistake.

Fixes https://github.com/sourcegraph/sourcegraph/issues/4079