Graceful failure of new_comment route

Fixes https://github.com/mysociety/alaveteli/issues/662

If /annotate/request/:url_title is accessed when comments are disabled
an exception is incorrectly thrown.

Conditionals should be used for control flow, so now the action
redirects to the info_request path and displays a notice.
Extract find_info_request from CommentController

Use a before_filter to make @info_request available to all filters
called on the same action
Extract create_track_thing from CommentController

Use a before_filter to make @track_thing available to all filters
called on the same action and remove responsibility from the #new method
Use filter to reject unless comments allowed

Extract checking whether comments are allowed on an InfoRequest to a
filter in CommentController.

Removes responsibility from the #new method.
Use filter to reject if user is banned

Extract checking whether a user is banned from making Comments on an
InfoRequest to a filter in CommentController.

Removes responsibility from the #new method.

Adds a missing spec.