[Commit 2] Add some models
[Commit 3] Create token, role and permission model, and move grader_configuration out of namespace grader
[Commit 4] Create models
[Commit 5] Add models specifications
[Commit 6] Modify models specifications, create migrations
[Commit 7] Enable Jbuilder, change bcrypt-ruby to bcrypt, implement login controller, create migration for grader configuration, edit seed for `root`