Implement autorespond feature

[MatthewDorner]

This is a duplicate of my PR which keeps getting broken since the repo was taken private and then back to public: #126. My mattermost-plugin-mscalendar repo still shows itself as being forked from cpurta/mattermost-plugin-msoffice so it's still broken, and I'm making this PR from my new fork in case you want to just merge this one or if the other breaks again.

[codecov-commenter]

Codecov Report

Merging #188 into master will decrease coverage by 0.04%.
The diff coverage is 21.83%.

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
- Coverage   23.52%   23.48%   -0.05%     
==========================================
  Files          62       64       +2     
  Lines        2516     2602      +86     
==========================================
+ Hits          592      611      +19     
- Misses       1850     1913      +63     
- Partials       74       78       +4     

Impacted Files	Coverage Δ
server/command/auto_respond.go	0.00% <0.00%> (ø)
server/command/command.go	41.11% <0.00%> (-0.94%)	⬇️
server/command/help.go	0.00% <0.00%> (ø)
server/mscalendar/mscalendar.go	100.00% <ø> (ø)
server/mscalendar/settings.go	0.00% <0.00%> (ø)
server/mscalendar/welcome_flow.go	0.00% <0.00%> (ø)
server/mscalendar/auto_respond.go	61.29% <61.29%> (ø)

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[larkox]

Thank you very much @MatthewDorner ! I think we can safely close the other PR, and continue with this one, in case any other change is needed (I hope not!).

[larkox]

Approving as previous PR was already approved.

[larkox]

@DHaussermann @mickmister Merging this PR since the original one was already approved by all of us, and was ready to merge.