Tags (and tag groups) can be configured so that they can only be used in specific categories and (optionally) restrict topics in these categories to be able to add/use only these tags. These restrictions work as expected when a topic is created without going through the review queue; however, if the topic has to be reviewed by a moderator then these restrictions currently aren't checked before the topic is sent to the review queue, but they're checked later when a moderator tries to approve the topic. This is because if a user manages to submit a topic that doesn't meet the restrictions, moderators won't be able to approve and it'll be stuck in the review queue.
This PR prevents topics that don't meet the tags requirements from being sent to the review queue and shows the poster an error message that indicates which tags that cannot be used.
Internal ticket: t60562.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
