Move repo tags to own controller. add ability to remove tags

app/controllers/projects/branches_controller.rb

app/controllers/projects/repositories_controller.rb

app/controllers/projects/tags_controller.rb

app/models/event.rb

app/models/repository.rb

app/views/projects/branches/_branch.html.haml

app/views/projects/commits/_head.html.haml

app/views/projects/tags/index.html.haml

config/routes.rb

lib/gitlab/backend/shell.rb