Group members Vue conversion - avatar component

What does this MR do?

• See #235605 (closed)
• Part of &4233 (closed)

Behind the vue_group_members_list feature flag. To enable:

1. bin/rails console
2. Feature.enable(:vue_group_members_list)

As part of the effort to convert the group members view from HAML to Vue this MR adds a member_avatar.vue component. To keep MRs small the rest of the table will come in a series of subsequent MRs.

Review app: https://gitlab-review-235605-con-n1ys7b.gitlab-review.app/groups/haml-to-vue-conversion/-/group_members

Screenshots

Desktop

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Mobile

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**Will be fixed by gitlab-ui!1735 (merged)**

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
  • Behind a feature flag, not needed
• [-] 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