Force tags to lowercase

MongoDB stores tags as case sensitive. Elasticsearch queries are case
insensitive. For consistency we are forcing all tags to lowercase. A
migration script is included to downcase existing tags.
Scope tags controller by account

The TagsController wasn't a subclass of the SubdomainController. It
failed to scope by Account.current_id.
Parameterize tags

Turns out there are more problems caused by malformed tags. For example,
including an exclamation point causes all sorts of fun. So this update
forces all tags to only use lowercase letters, numbers and hyphens.
Tag modification

Edit tag allows merging, renaming, splitting, and deleting tags.
TagsController does the heavy lifting. It can be quite slow for large
tag counts.