When a user subscribes, ensure they have a default notifier configured

[kfdm]

This should reduce confusion if a user subscribes to a project/service but does not already have notifications subscribe. It will default to using owner.email to create a notification or print a warning if it does not exist

[codecov-io]

Codecov Report

Merging #122 into master will decrease coverage by 0.26%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #122      +/-   ##
=========================================
- Coverage   44.77%   44.5%   -0.27%     
=========================================
  Files          37      37              
  Lines        2372    2386      +14     
=========================================
  Hits         1062    1062              
- Misses       1310    1324      +14

Impacted Files	Coverage Δ
promgen/signals.py	27.45% <0%> (-2.34%)	⬇️
promgen/views.py	43.75% <0%> (-0.11%)	⬇️

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