Add UserHasBeenDeactivated hook to disconnect user

[MatthewDorner]

Summary

Implement UserHasBeenDeactivated plugin hook to disconnect a user when that user is deactivated.

Ticket Link

Fixes #287

Maybe obvious, but in order to test, you have add to the plugin's go.mod: replace github.com/mattermost/mattermost-server/v6 v6.5.0 => ../mattermost-server to point to your local copy of server with the changes from my server PR: mattermost/mattermost#20894

[mattermod]

Hello @MatthewDorner,

Thanks for your pull request! A Core Committer will review your pull request soon. For code contributions, you can learn more about the review process here.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.04% ⚠️

Comparison is base (835b440) 32.94% compared to head (ea61627) 32.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #315      +/-   ##
==========================================
- Coverage   32.94%   32.91%   -0.04%     
==========================================
  Files          22       22              
  Lines        4013     4017       +4     
==========================================
  Hits         1322     1322              
- Misses       2560     2564       +4     
  Partials      131      131              

Files Changed	Coverage Δ
server/plugin.go	14.39% <0.00%> (-0.09%)	⬇️

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[mattermod]

This PR has been automatically labelled "stale" because it hasn't had recent activity.
A core team member will check in on the status of the PR to help with questions.
Thank you for your contribution!

[MatthewDorner]

@mickmister @ashishbhate Updated here to match the updated plugin hook function signature in the server PR.

[hanzei]

Neat 🎉

[mickmister]

Maybe obvious, but in order to test, you have add to the plugin's go.mod: replace github.com/mattermost/mattermost-server/v6 v6.5.0 => ../mattermost-server to point to your local copy of server with the changes from my server PR: mattermost/mattermost#20894

Note that this means we have to adjust this project's go.mod to point the appropriate server version after the new server version is released

[spirosoik]

nice 🎉

[mattermod]

This PR has been automatically labelled "stale" because it hasn't had recent activity.
A core team member will check in on the status of the PR to help with questions.
Thank you for your contribution!

[hanzei]

@DHaussermann Gentle reminder to review this PR

[mattermost-build]

This PR has been automatically labelled "stale" because it hasn't had recent activity.
A core team member will check in on the status of the PR to help with questions.
Thank you for your contribution!

[DHaussermann]

/update-branch

[hanzei]

#393 needs to get merged first.

[hanzei]

Tested and confirmed that users who get deactivated also disconnect from GitLab. Ready for merging 🚀

[hanzei]

Thanks, @MatthewDorner, for the contribution. Sorry, it took so long.