ref(alerts): Move anomaly detection code out of subscription processor

[ceorourke]

This PR moves anomaly detection specific functions out of the subscription processor, there are no logic changes.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.05970% with 8 lines in your changes missing coverage. Please review.

:white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
.../sentry/seer/anomaly_detection/get_anomaly_data.py	86.00%	5 Missing and 2 partials :warning:
src/sentry/incidents/subscription_processor.py	85.71%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #78525      +/-   ##
==========================================
+ Coverage   78.11%   78.15%   +0.04%     
==========================================
  Files        7078     7093      +15     
  Lines      312081   312872     +791     
  Branches    50973    51109     +136     
==========================================
+ Hits       243769   244528     +759     
- Misses      61954    61959       +5     
- Partials     6358     6385      +27