Improve UX when asserting post thresholds

While the post thresholds are still validated on submission, the tester of the linked QA test brought up an issue with regards to UX of this. For e.g. with the inline forms, you can still enter content for a reply OR a new discussion. The user is only notified of the threshold once they submit forms. At this point(specifically when adding a new discussion), the content entered is lost when you navigate back to the form.

Compared to 3.6, a notification gets displayed immediately since we navigate away from the page and the threshold check blocks any entry to be commenced.

Pinging barbararamiro and ashbettridge for some feedback