Fix Jira DVCS user avatar path

What does this MR do?

Related to #233016 (closed)

We didn't include the full path to self-hosted avatars in our v3/github API. Those endpoints are used to connect with a Jira instance via DVCS connectors. This was leading to broken images.

Screenshots

How to test on staging

1. Create a user with and upload an avatar
2. Set up Jira Devevelopment Panel via DVCS
3. Create an MR and reference a Jira issue in the title
4. Go to Jira and sync the repository
5. Check if the avatar is displayed

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