[INTERNAL] Move remaining celery tasks to tasks.py

[kfdm]

More cleanly split tasks.py and prometheus.py in preparation for additional cleanup and refactoring

[codecov-io]

Codecov Report

Merging #139 into master will increase coverage by 0.32%.
The diff coverage is 15.38%.

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
+ Coverage   46.63%   46.95%   +0.32%     
==========================================
  Files          41       41              
  Lines        2464     2462       -2     
==========================================
+ Hits         1149     1156       +7     
+ Misses       1315     1306       -9

Impacted Files	Coverage Δ
promgen/signals.py	27.67% <0%> (+0.34%)	⬆️
promgen/views.py	48.28% <0%> (ø)	⬆️
promgen/prometheus.py	67.3% <0%> (+9.69%)	⬆️
promgen/tasks.py	39.72% <17.77%> (-60.28%)	⬇️
promgen/celery.py	73.91% <0%> (+69.56%)	⬆️

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