feat(spotlight): Inject Spotlight button on Django

[BYK]

This patch expands the SpotlightMiddleware for Django and injects the Spotlight button to all HTML responses when Spotlight is enabled and running. It requires Spotlight 2.6.0 to work this way.

Ref: getsentry/spotlight#543

[codecov]

Codecov Report

Attention: Patch coverage is 44.82759% with 32 lines in your changes missing coverage. Please review.

Project coverage is 84.29%. Comparing base (d424226) to head (44552a9).
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	44.82%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3751      +/-   ##
==========================================
- Coverage   84.40%   84.29%   -0.12%     
==========================================
  Files         136      136              
  Lines       14379    14418      +39     
  Branches     2422     2429       +7     
==========================================
+ Hits        12137    12154      +17     
- Misses       1491     1514      +23     
+ Partials      751      750       -1     

Files with missing lines	Coverage Δ
sentry_sdk/spotlight.py	58.33% <44.82%> (-9.79%)	⬇️

... and 3 files with indirect coverage changes

[sentrivana]

lgtm, left some suggestions/questions