Remove the TagRepository
Remove unused and `Tag.id_and_entity` method.
Use methods on Tag to load tags and tag lists.

Mostly this just factors out the common occurrences of `Tag.where(...).first`
in the codebase, but it also lets us load a list of tags with a single Mongo
query using the `Tag.by_tag_ids` method or its corresponding bang method.
Use `for_tag_ids` in Taggable for loading.
Merge pull request #43 from alphagov/remove_tag_repository_rebased

Replace the tag repository