Simplify user popover loading logic

What does this MR do?

See #214125 (closed)

Simplifies loading logic in app/assets/javascripts/vue_shared/components/user_popover/user_popover.vue

Screenshots

Before

Screen_Recording_2020-07-02_at_11.43.29_AM

After

Screen_Recording_2020-07-02_at_11.40.12_AM

*Network speed was throttled in dev tools to show loading indicators for longer

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry

  • Any developer-facing change (e.g., refactoring, technical debt remediation, test suite changes) should not have a 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