Mainly modify for seeding users with school and groups

- Add teacher seeds from file
- develope V1::Dashboards::Admin::UsersController from authorization to rendered json
- modify `user` <-> `group` relations to `users` <-> `ugroups`
- add enrollment relations so `user` could have several `groups`
- add `add_group` `enroll` to simplify user creation
- add namespace authorization for cancan
shorthand for create roles

- replace User.mock with User.new in user.seeds.rb to solve empty role issue
fix sqlite3 transaction error
add menu#update for admin
Merge pull request #12 from Bratops/0.1.9_user_mgt

User management - menu support