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