What type of PR is this? (check all applicable)

• [x] Feature

Description

This PR completes the final piece of functionality to fully ship RFC #22! 🥰

Per the RFC's design/definition of done, this PR does the following:

• [x] Allows articles to have mentions
• [x] Sends users who are mentioned in an article a single notification
• [x] Ensures that the correct notification shows up on the mentioned user's /notification page
• [x] Ensures that a user only receives one notification per article (if a user follows an author/organization but also happen to be @-mentioned in the article, they only receive the @-mention notification, and no other article notifications)
• [x] Updates relevant views and mailer templates
• [x] Adds a TON of specs

Related Tickets & Documents

Completes https://github.com/forem/rfcs/pull/22! 🎉

QA Instructions, Screenshots, Recordings

To QA this PR, open up two browser windows, each with two different users. In this case, I have User A (Admin user) and User B (my user, vaidehijoshi).

Part 1: Testing @-mentions while following a user

1. Start by having User B follow User A. In User A's account, create an article, and mention User B:

2. Publish the article written by User A:

3. Go to User B's account, and check that you see a notification icon:

4. Click on the icon, which takes you to the `/notifications` tab. You should see a notification that tells you that you were mentioned. You SHOULD NOT have received a notification telling you that User A published the article (which you normally would get if you follow User A). Instead, you should _only_ have a notification telling you that you were mentioned in the new post they published:

5. Clicking on the notification itself should take you to the newly-published article, and you should see your username mentioned 👍🏽

Part 2: Testing normal article published notifications

1. Now, create a new article from User A's account. This time, DO NOT mention User B.

2. In User B's account, check that you see a notification icon:

3. You should see a normal "article published" notification:

Part 3: Testing embedded mentions

1. Find a comment that mentions User B (any comment should do!). For example:

2. Create a new post from User A's account, using the `{% comment [id] %}` liquid tag syntax:

3. Go to User B's account; if User B is still following User A, there should be a new notification, but it should NOT be a notification about being @-mentioned. Instead, it should only be a normal "article published" notification, and nothing else.

4. You can try the same test after making User B "unfollow" User A, at which point User B should receive NO notifications.

Part 4: Other oddities

1. Try testing this functionality out with both editors; the functionality should be exactly the same:

2. Test that a user doesn't get more than one notification, no matter how many times they are mentioned! ALso test that you can't @-mention more than 7 times:

3. Test that you can [preview the new `new_article_mention_email` mailer](http://localhost:3000/rails/mailers/notify_mailer) locally, and that the job to send the email is enqueued when a mention notification is sent locally.

UI accessibility concerns?

This is mostly backend work, and the frontend work is reusing HTML markup code that already exists so, hopefully not!

Added tests?

• [x] You bet I did 😉

[Forem core team only] How will this change be communicated?

• [x] I will share this change in a Changelog or in a forem.dev post
• [x] I will share this change internally with the appropriate teams

Are there any post deployment tasks we need to perform?

Nothing special, nope! Just want to be sure to QA thoroughly and try it out on DEV before rolling it out to the fleet.

What gif best describes this PR or how it makes you feel?