Add Tags Section to Admin

[MitchellMcKenna]

• Inside this section there is a paginated list of all the tags.
• Sorted by popularity by default, but sort by name as well.
• Ability to see those tags which aren't assigned to a public posts.
• Button to force cleanup(); which removes tags not assigned to any posts (then item_model::clean_tags() will not have to run as often).
• Tags can be deleted and all items with that tag will have that relationship removed.
• Ability to change the name or slug of a tag.
• Tags can be merged, during a merge user can specify name and slug.
• Maybe even ability to map a tag found in future posts to be tagged with another tag instead.
• The tags list in the admin sidebar can then be removed or limited at 50 (as this list can get unwieldy and cause long page load times in the admin section).

Refs #38