User's blocked field refactored to use state machine

app/controllers/admin/users_controller.rb

app/controllers/application_controller.rb

app/models/user.rb

app/views/admin/users/_form.html.haml

app/views/admin/users/index.html.haml

app/views/admin/users/show.html.haml

app/views/team_members/_team_member.html.haml

app/views/teams/members/_show.html.haml

db/schema.rb

lib/api/entities.rb

lib/gitlab/auth.rb

spec/models/user_spec.rb