Fix Slack slash commands using relative URL

What does this MR do?

As reported in this comment: gitlab-mattermost#101 (comment 310769792), Slack and Mattermost slash commands integration sometimes uses the author avatar with a relative path which causes the image to show up as broken when displayed from a different domain (most of the time).

This MR adds an absolute link to the same.

Screenshots

Integration	Before	After
Slack
Mattermost

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
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• 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