Implement bulk download for missions
[migrations] remove and fix invalid migration files

db:migrate run without error for empty database
[Migration] remove old_tags table (no longer in use)
Merge pull request #202 from wangqiang1208/buggfixing/bulk-download

Fix bulk download for missions
Merge branch 'development' of github.com:Coursemology/coursemology.org into development
Merge branch 'development' of github.com:Coursemology/coursemology.org into Deploy