Use siteurl on webapp to avoid issues with subpath

[mickmister]

Ticket Link

Fixes #176

[codecov]

Codecov Report

Merging #177 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #177   +/-   ##
=======================================
  Coverage   39.89%   39.89%           
=======================================
  Files          16       16           
  Lines        1494     1494           
=======================================
  Hits          596      596           
  Misses        838      838           
  Partials       60       60           

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[hanzei]

@mickmister Heads up that you have some linter issue: https://app.circleci.com/pipelines/github/mattermost/mattermost-plugin-gitlab/399/workflows/b38fd557-0d34-4d5f-be19-2b3085ef1f50/jobs/1208/parallel-runs/0/steps/0-106

[iomodo]

LGTM! 👍

[larkox]

LGTM

[DHaussermann]

@iomodo if you have time, is this something you would be able to peer test using the peer testing process documented here?https://mattermost.atlassian.net/wiki/spaces/INT/pages/619937892/Peer+Testing+Process

[iomodo]

@DHaussermann tested, works as expected.

[mickmister]

@DHaussermann Good to merge this?

[DHaussermann]

Huge thanks to @iomodo for testing on this
I added a test for this to release testing
I Will include this is release testing for the next GitLab release
LGTM!