feat(django): Add SpotlightMiddleware when Spotlight is enabled

[BYK]

This patch replaces Django's debug error page with Spotlight when
it is enabled and is running. It bails when DEBUG is False or
when it cannot connect to the Spotlight web server.

[codecov]

Codecov Report

Attention: Patch coverage is 53.12500% with 15 lines in your changes missing coverage. Please review.

Project coverage is 84.27%. Comparing base (2bfce50) to head (344af85).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/spotlight.py	51.61%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3600      +/-   ##
==========================================
- Coverage   84.34%   84.27%   -0.07%     
==========================================
  Files         133      133              
  Lines       13994    14024      +30     
  Branches     2951     2955       +4     
==========================================
+ Hits        11803    11819      +16     
- Misses       1450     1465      +15     
+ Partials      741      740       -1     

Files with missing lines	Coverage Δ
sentry_sdk/client.py	83.49% <100.00%> (ø)
sentry_sdk/spotlight.py	65.57% <51.61%> (-15.08%)	⬇️

... and 3 files with indirect coverage changes