Fix to allow modified files to be excluded

[leonsodhi-lf]

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.

[codecov]

Codecov Report

Merging #847 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #847      +/-   ##
==========================================
+ Coverage   71.83%   71.88%   +0.04%     
==========================================
  Files          65       65              
  Lines        5213     5221       +8     
==========================================
+ Hits         3745     3753       +8     
  Misses       1183     1183              
  Partials      285      285

Impacted Files	Coverage Δ
server/events/project_finder.go	94.38% <100%> (+0.55%)	⬆️

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[lkysow]

I carried your commits to #865. Thanks for the great fix!