Draft: Load user popover dependencies only when necessary

What does this MR do?

This loads user_popovers.js's dependencies only when there is a .js-user-link. This is a ~40kb reduction in main.js, see bundle size analyzis.

Update The performance statistics indicate that this MR would increase performance on the Project and Issue page, but degrade performance on a MR page. More on that in the following thread: !47677 (comment 457547530). We've decided to not merge it at the current state, because we don't see an improvement on all pages, and especially the MR page is the slowest of these three.

Why is this not improving performance? A few thoughts:

• Before this MR, the user_avatar was loaded in main.js bundle with user_popover, which would be no longer the case and needs to be loaded on every page separately
• With this MR, we generate a initPopover and a vendor-initPopover chunk. It would be interesting to know why webpack does this and if it would be valuable to prevent

Ideas how to improve this MR:

• Maybe it's worth putting user_avatar back in main.js
• Maybe we can only load user_popover on mouseover instead of when a .js-user-link class is present.

Screenshots (strongly suggested)

User Popover on page	User Popover on markdown preview

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #246533