Anchor the appsbar icon to SiteURL

[lieut-data]

Summary

This ensures the icon can be correctly loaded when a subpath is configured and the application isn't running at the root.

Ticket Link

Fixes: #343

[codecov]

Codecov Report

Base: 33.32% // Head: 33.32% // No change to project coverage 👍

Coverage data is based on head (9eccd41) compared to base (09907ab).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #347   +/-   ##
=======================================
  Coverage   33.32%   33.32%           
=======================================
  Files          21       21           
  Lines        2785     2785           
=======================================
  Hits          928      928           
  Misses       1748     1748           
  Partials      109      109           

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[DHaussermann]

@lieut-data for this PR I do not see the icon working on our subpath test server.

The URL is set to https://subpath.test.mattermost.com/mattermost as expected

My first though is that this solution is dependant on your server side code here mattermost/mattermost#21886 is this correct?

If so, I can't test this PR right away as I have no reliable way of getting the server side code change onto a subpath setup right now.

That being said I did regression test this change on other servers and see no issue with the App Bar icon display.

Do you have any thoughts on merging this now to include in the next release?

[lieut-data]

@DHaussermann, yes this PR depends on mattermost/mattermost#21886. Assuming it doesn't regress as you've identified for non-subpath servers, I'm 👍 for merging now.

[DHaussermann]

Tested and passed

• Regression testing was done on this PR with no impacts found.
• Plugin icons for App Bar on sub-path servers will not be supported until Mattermost v7.8.0
• Once I have the infrastructure in place, I will test and create a new issue if any further changes are needed to support App Bar icons on sub-path servers for GitLab

LGTM!
Thanks @lieut-data for the effort on this and the broader solution.

Please merge.