Move groups models from users app, add groups index, add stubs for create groups
Fix registration empty user_id bug
Merge branch 'develop' into group_management

fix registration bug
Add view and validation for create group
Add delete group feature
Add group edit view and delete user from group
Add API for search users by username
Merge branch 'develop' into group_management
Add user adding to group, modify user search API for including avatars and fullnames
Merge branch 'group_management' into develop

Issue #2
Merge branch 'develop'