Add avatar images for newly created Support Bot and Alert Bot

What does this MR do?

Fixes one part of #210523

Avatar images have been obtained from #210523

This change sets the custom avatars for newly created Alert and Support Bots.

For these bots, the avatar attribute will now point to the path of the image on disk, so that, on saving the record, the avatar gets uploaded as their avatar image.

The next part would be to have a data-migration to change the avatars of existing Alert and Support Bots (in .com and in existing self-managed instances)

Screenshots

Before	After

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
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