Prevent group deletion for groups referenced in AutoGroups configuration

[stjosh]

Closes #28

[codecov]

Codecov Report

Merging #34 into master will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master       #34   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity        15        17    +2     
===========================================
  Files              5         5           
  Lines             49        58    +9     
===========================================
+ Hits              49        58    +9     

Flag	Coverage Δ	Complexity Δ
#master	100.00% <100.00%> (ø)	17.00 <12.00> (+2.00)
#stable18	100.00% <100.00%> (ø)	17.00 <12.00> (+2.00)

Impacted Files	Coverage Δ	Complexity Δ
lib/AutoGroupsManager.php	100.00% <100.00%> (ø)	12.00 <12.00> (+2.00)

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