Remove ability to update a tag on success

Removes the use_branches_on_green column from repositories and defaults the behavior to updating a branch. This options was originally added to allow developers to choose between updating  a tag or updating a branch with the latest green ref on the master branch. In retrospect, using tags for this is terrible since they are not meant to be updated.

This PR also contains a few style changes to the repository edit form.

@square/kochiku-contributors