Provide a plaintext response to stats submissions.

Provide a plaintext response to stats submissions.

For all of XonStat's history the POST requests sent in by servers had plain
HTTP responses. This begins to change with this commit, which responds back to
the Xonotic server with meaningful information about /what happened/ during the
match that was just submitted. Of particular importance are the player_ids and
elo delta values that were stored as a result of the game. The template
establishing the format for this is submit_stats.mako. It will be extended
after we decide what other things we want to include.