feat: send notification when license limit was exceeded

[DeepDiver1975]

ToDo:

• [ ] define proper notification subject (the bold title line)
• [ ] define notification message

[codecov[bot]]

Codecov Report

Merging #10 (e133b75) into master (d3d2af9) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff            @@
##             master     #10   +/-   ##
========================================
  Coverage      0.00%   0.00%           
- Complexity       53      61    +8     
========================================
  Files             4       7    +3     
  Lines           156     197   +41     
========================================
- Misses          156     197   +41     

Impacted Files	Coverage Δ	Complexity Δ
appinfo/app.php	0.00% <0.00%> (ø)	0.00 <0.00> (?)
lib/Application.php	0.00% <0.00%> (ø)	2.00 <2.00> (?)
lib/Command/CheckActiveUsers.php	0.00% <0.00%> (ø)	23.00 <3.00> (+2.00)
lib/Notifier.php	0.00% <0.00%> (ø)	4.00 <4.00> (?)

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