Disable markdown folding on gitlab < 11.1

[lkysow]

On startup, make a call to the /version endpoint of Gitlab to get which
version we're running. Then use that version to determine whether we
should use the markdown folding syntax where the code is expandable.
Versions of Gitlab < 11.1 don't support the "Common Mark" markdown
format.

Fixes #315

[codecov]

Codecov Report

Merging #366 into master will decrease coverage by 0.05%.
The diff coverage is 43.63%.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
- Coverage    70.6%   70.54%   -0.06%     
==========================================
  Files          61       61              
  Lines        3681     3718      +37     
==========================================
+ Hits         2599     2623      +24     
- Misses        901      908       +7     
- Partials      181      187       +6

Impacted Files	Coverage Δ
server/events/markdown_renderer.go	92.77% <100%> (+0.17%)	⬆️
server/server.go	66.01% <33.33%> (+3.32%)	⬆️
server/events/vcs/gitlab_client.go	20.4% <42.55%> (+20.4%)	⬆️
server/logging/simple_logger.go	84.28% <0%> (+1.42%)	⬆️

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