Store group members in matches table

[cheshire137]

Fixes #62.

• Add a new group_member_ids array field to matches table and populate it based on the match_friends table
• Drop MatchFriend model but leave the table for now
• Make use of group_member_ids to store the group members in each match

This will help keep row counts down: I can log a match where I played with 4 friends and only have 1 row created instead of 5, assuming friends records already exist. It will also improve query performance when trying to load all group members for all matches in a season.

[cheshire137]

To compare before/after output locally:

psql competiwatch_development -c "SELECT match_id, ARRAY_AGG(friend_id ORDER BY friend_id) AS friend_ids FROM match_friends GROUP BY match_id ORDER BY match_id;" > before-local.txt
rake db:migrate
psql competiwatch_development -c "SELECT id AS match_id, group_member_ids AS friend_ids FROM matches WHERE group_member_ids <> '{}' ORDER BY match_id;" > after-local.txt
diff before-local.txt after-local.txt

[cheshire137]

Similar results for production:

heroku pg:psql -c "SELECT match_id, ARRAY_AGG(friend_id ORDER BY friend_id) AS friend_ids FROM match_friends GROUP BY match_id ORDER BY match_id;" > before-prod.txt
heroku run rake db:migrate
heroku pg:psql -c "SELECT id AS match_id, group_member_ids AS friend_ids FROM matches WHERE group_member_ids <> '{}' ORDER BY match_id;" > after-prod.txt
diff before-prod.txt after-prod.txt