Feature: user can now request editor access to dataset he/she doesn't
own.

Details:

* Button in dataset page: request access.

* Green when access request is not pending, red if pending.

* Click triggers an email to owner.

* Email contains a link to controller action unlock_access, which gives
the required editor rights via CKAN authz. Also removes access request.

* If the link points to no available access request, a redirect to / is
made.