Adds ticket tagging for admin users.

Admins may now add tags to tickets. The tickets may also be searched
by tag in the Tickets#index as well provided that the current_user is
an admin. The idea here is that we want to let admins add arbitrary
labels to tickets to make organizing tickets easier. The need for tags
isn't as great from a regular User's perspective since they will have
a smaller overal number of tickets to begin with, and can already
categorize them by Department, Priority & Status.

I've only added the bare minimum for Tags to allow admins to tag
tickets. So at this point that only constitutes creating a tag. Edit,
delete, and other functionality will follow later.