Add admin tag pages for taggable records

[garethrees]

It would be useful to have:

• /admin/bodies/tags
• /admin/requests/tags
• etc

To serve a few purposes:

• Make it clear what tags even exist (Agree, document, and make consistent, our use of request tags  whatdotheyknow-theme#1158)
• Provide some stats on how many times each tag is used (e.g. Obtain a list of request tags in-use along with their frequency of use whatdotheyknow-theme#1159)
• Link to searches for content tagged with a given tag (e.g. click tag takedown to see all tagged records of the current taggable with the takedown tag)

A basic listing would be a good first step, but additional niceties would be:

• search
• filters (sort by name, most used)
• a both_links implementation so that we get <eye> tag, where the eye links to a public search for the tag and the tag links to an admin search for the tag.
• A way of finding all taggables with the given tag (I guess a link to search at an overall /admin/tags page)