Switch to a different gem for rendering MediaWiki markup

This one has some small modifications to make it act more like PHP's
markup.

Fixes #195
Mark a PublicWhip specific attribute as OK

This is mainly to make PHP and Rails align, it also can't hurt
(famous last words!).
Add ability to find by MP IDs, as TWFY/OA links do
Re-enable tests now that #195 is fixed
Use policy ID supplied in params if present. Fixes #226
The policy ID is also used in the form name/id
Respect the policy ID specified in the query string when saving vote

Fixes #226 (again)
Remove extra '/' - the path always contains a leading slash
Actually use the modified parameter name added in f244cd2
Add a test for removing a division
Redirect account registration to devise
Merge pull request #228 from DanielKinsman/issue227

Redirect account registration to devise