Add assignees to boards sidebar

What does this MR do?

This MR adds assignees to the epic boards sidebar. Because the existing functionality, specifically with the dropdown, its tightly coupled to Jquery. This gives us a basic read only we can build off of, reusing as much as we can.

There are some style updates that need to happen.

• Link color to black, not the default blue.

Repro:

• Turn on FF :graphql_board_lists AND :boards_with_swimlanes
• Go to Epics -> Boards -> Switch to Epics
• Click on a card and notice that the sidebar contains a read only view of assignees

Non trivial changes that needed to happen on this MR:

• assignee_avatar_link.vue
  • Needed to add the option of webUrl to be returned to avoid massaging data from Graphql endpoints
• uncollapsed_assignee_list.vue
  • Replaced spans with divs so that content splits on 2 lines in the sidebar regardless of width. No visual regressions should happen in any of the sidebar. Because of this change, we were able to remove specific scss.

Screenshots

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