Allow when_modified files to be excluded

[lkysow]

I noticed that setting when_modified to something like ["*.tf", "!exclude-me.tf] with a modified list of exclude-me.tf still resulted in a plan automatically running. This PR allows files to be excluded using .dockerignore's exclusion syntax, i.e. prepending a ! to the path.

(from #847)

Closes #847 (rebased those commits).

[codecov]

Codecov Report

Merging #865 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #865      +/-   ##
==========================================
+ Coverage   71.82%   71.88%   +0.05%     
==========================================
  Files          65       65              
  Lines        5214     5221       +7     
==========================================
+ Hits         3745     3753       +8     
+ Misses       1184     1183       -1     
  Partials      285      285

Impacted Files	Coverage Δ
server/events/project_finder.go	94.38% <100%> (+0.55%)	⬆️
server/events/vcs/azuredevops_client.go	73.12% <0%> (-0.93%)	⬇️
server/events/yaml/raw/global_cfg.go	0% <0%> (ø)	⬆️
server/events/event_parser.go	85.92% <0%> (ø)	⬆️

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