assets precompile
added preference settings for mission and training paging
home page preference settings, leaderboard setting, announcements paging setting
add grading last update time, disallow multiple gradings for mission.
fix bug when mission change format
round up grade to 5, added multiplier
show answer explanation and review for select-all mcq
mission unsubmit - not safe, doesn't refresh exp.
Merge branch 'development' of github.com:Coursemology/coursemology.org into dev/raymoond