#209: Add ability to link a vote to bill from vote page

Implement functionality as described in #209.
Includes tests for possible scenarios, ~~*does not* include browser-tests.~~ including browser tests.

Includes a change to the `slugify_name` function so that it will handle unicode better.

NOTE:
The form will appear for all authenticated users (`user.is_authenticated()`).
Is there any other restriction necessary?
This restriction is similar to the one used on the bill page to attach a vote.