ref(demo-mode): extract to module

[obostjancic]

• Closes https://github.com/getsentry/projects/issues/737
• Moves demo_mode out of utils and into its own module. Does not add any new functionality.

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/api/permissions.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #86377      +/-   ##
==========================================
+ Coverage   85.39%   87.84%   +2.44%     
==========================================
  Files        9746     9736      -10     
  Lines      552308   552169     -139     
  Branches    21537    21529       -8     
==========================================
+ Hits       471669   485069   +13400     
+ Misses      80085    66714   -13371     
+ Partials      554      386     -168