Admins can currently add the bookmarks discovery route link
to the homepage interface, but users can't presently select
that as their default home view. This change facilitates that,
adding the option to the existing Default Home Page dropdown on
the User Preferences Interface page.
* FEATURE: add category banner for why a user cannot post
Adds a category banner for why a user is unable to post in a category.
Also adds an extra alert for the user when a user is unable to create a topic in a
category and they still try and click on the disabled-looking new topic
button.
