Tag mappings

Only addresses single mapping tagging and adding tags while bulk adding as per the [story](https://www.pivotaltracker.com/story/show/64218782).

Bulk tagging of existing mappings is now [separated out](https://www.pivotaltracker.com/story/show/64841842).

(You can say that 5734b7aa0019aeeeb42cc3bdfc027d8c4e5add12 also deserved a separate PR. You'd get no argument from me)