chore(insights): Add feature flag and referrers for mobile vitals module

[markushi]

Adds feature flag definition, so we can toggle the feature via flagpole. Also adds referrers which can later be used in the frontend queries.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.23%. Comparing base (668079e) to head (1dd8b8d). Report is 71 commits behind head on master.

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #75428      +/-   ##
==========================================
- Coverage   78.24%   78.23%   -0.02%     
==========================================
  Files        6819     6804      -15     
  Lines      303711   303423     -288     
  Branches    52239    52187      -52     
==========================================
- Hits       237651   237377     -274     
+ Misses      59669    59640      -29     
- Partials     6391     6406      +15     

Files	Coverage Δ
src/sentry/api/endpoints/organization_events.py	85.02% <ø> (ø)
src/sentry/features/temporary.py	100.00% <100.00%> (ø)
src/sentry/snuba/referrer.py	99.62% <100.00%> (+<0.01%)	:arrow_up:

... and 397 files with indirect coverage changes