MergeRequest#show performance improvements

This is a first pass on improving the performance of the MergeRequests#show page. Notable changes:

• The "Commits" tab is loaded lazily, so the initial page load should be much faster for MRs with many commits.
• Relative timestamps via timeago are only initialized once per load instead of O(n^2). This greatly improves frontend rendering times for a large number of commits.
• Refactored User.find_for_commit to use a single ARel-generated SQL query instead of the old method which resulted in one query, and could result in up to three.