Add a tag administration page (for admins only)

[braid-sync]

A task for new Braid contributors.

Goal:
To create a new page, for group admins, to be able to easily administer existing tags.
It should list all tags (with # of threads they are used it) and allow renaming. It should show a list of tags without any threads and allow deleting (this could be done as a second PR)

Notes:

• This should be implemented a separate module
• braid.stars is a good reference for how to do modules
• braid.core.api already has a number of hooks that may be useful, (ex. "creating a group page"), but a few new ones may need to be added (ex. "adding item to admin menu")
• the user subscription management page may be a useful reference (braid.core.client.ui.views.pages.tags)