allow admins to close user accounts from the backend

* thanks to @maxwell for the initial work on this

port admin pages to bootstrap
* improve user view on admin search page
* add 'close account' link to each user in the search results
* keep the same blue color for the admin menu

some refactoring of the routes and the admin code
* try to be more RESTful (possibly)
* use a 'UserSearch' model for search parameters and querying

add changelog entry