Allow (un)following someone in the user popover

What does this MR do and why?

This MR is a suggestion to make it possible for users to follow and unfollow users via the user popover on notes, issues, and more. This makes it easier to follow people and reduces the friction of having to go to the user's profile. It also allows you to quickly determine whether you are following someone.

The (un)follow actions are optimistically atomic and will change instantly after the user clicks on the button. If an error occurs, the button is changed back.

Inspired by #346623 and Twitter's user popover.

Screenshots or screen recordings

Before	Follow	Unfollow	Signed out or same user

How to set up and validate locally

1. Enable the feature flag (rollout issue) using the Rails console: bin/rails c

   Feature.enable(:follow_in_user_popover)

2. Go to an issue that you didn't create
3. Hover over the user name of the user who created the issue
4. See the follow/unfollow button
5. When you click it: see that it changes state and the user is

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

 /closes #353362 (closed)

👋 @KevSlashNull

Thank you for your contribution to GitLab. We believe that everyone can contribute and contributions like yours are what make GitLab great!

• Our Merge Request Coaches will ensure your contribution is reviewed in a timely manner*.
• If you haven't, please set up a DANGER_GITLAB_API_TOKEN.
• You can comment @gitlab-bot label ~"group::" to add a group label.
• After a few days, feel free to ask @gitlab-bot help or ping a Merge Request Coach.
• Read more on how to get help.

This message was generated automatically. You're welcome to improve it.

added Community contribution label

This MR will be considered part of the quarterly GitLab Hackathon for a chance to win a prize.

Can you make sure this MR is mentioning/linking the relevant issue that it's attempting to close?

Thank you for your contribution!

added Hackathon label

changed the description

added UX label

Thanks for helping us improve the UX of GitLab, your contribution is appreciated! We have pinged UX team members, stay tuned for their feedback.

Thanks for this contribution @KevSlashNull! 🙇

I suggest a designer from devopsmanage reviews this. @badnewsblair can you suggest someone?

added devopsplan sectiondev labels

added devopsmanage groupauthentication and authorization [DEPRECATED] labels and removed devopsplan label

mentioned in issue KevSlashNull/KevSlashNull#3 (closed)

Setting label(s) ~"Category:Authentication and Authorization" based on ~"group::access".

added 1 deleted label

requested review from @matejlatin

mentioned in issue gitlab-org/quality/triage-reports#5565 (closed)

added 847 commits

• 940e3328...9618f99c - 846 commits from branch gitlab-org:master
• 76f76b57 - Allow (un)following someone in the user popup

Compare with previous version

added 719 commits

• 76f76b57...4831b598 - 718 commits from branch gitlab-org:master
• 43d37b9b - Allow (un)following someone in the user popup

Compare with previous version

added 379 commits

• 43d37b9b...02917968 - 377 commits from branch gitlab-org:master
• 12e25752 - Allow (un)following someone in the user popup
• 1383483a - Add test for follow button

Compare with previous version

changed the description