Add `add ticket' to user admin page.

Add ability to add a ticket from the admin user edit page. Add nested
routes to handle creating a new ticket for a certain user -- previously,
though a route was defined, it didn't contain an id, so posting to the
`admin_ticket_create' path was difficult.

Added `new' and `create' actions to the tickets controller, create a new
ticket admin page, and add a button to add tickets in the admin user
edit page.
Update colors and text of ticket add /edit pages.

In the previous commit, the buttons were uniformly changed from `Add
User' in `btn-danger' to `Add user' in `btn-primary' in the user
add / edit pages. These changes have been mirrored for the ticket
add / edit pages.
Slight refactor of models and Mailchimp functions.

Rename `remove_mailchimp' to `unsubscribe_mailchimp' in ticket.rb.
Rename `mailchimp_unsubsctibe' to `unsubscribe_mailchimp' in user.rb.
Rename `mailchimp_subsctibe' to `subscribe_mailchimp' in user.rb.
Change order of `unsubscribe_mailchimp' and `subsctibe_mailchimp' in
ticket.rb.
Unsubscribe Mailchimp on ticket destruction.
Unsubscribe Mailchimp on user destruction.
Allow admins to create users with non-Oxford emails.

By adding an `external_email' field, we can distinguish between users
who have external emails, which were created by admins, and users who
have Oxford emails (whether created by an admin or not). The former will
be able to change their email as they like, since it skips the
college_regex email validation in the user model.

A user has an "external email" iff it was created by an admin, AND had a
non-Oxford email upon creation.