Initial code parts for CSP implementation for sentry and self-hosted

[oioki]

Another attempt of #47980 and #48507 (which were reverted due to bugs).

This PR adds some preliminary code for adding a Content-Security-Policy-Report-Only header with minimal required permissions (at least I could not find any violations on sentry devserver and self-hosted).

The CSP middleware is disabled (commented in the MIDDLEWARE)
There is no report collecting enabled by default (CSP_REPORT_URI is not set), the intent is to customize it depending on the use case.

[codecov]

Codecov Report

Merging #48699 (87a07b7) into master (3b3b9d7) will increase coverage by 0.60%.
The diff coverage is 93.93%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #48699      +/-   ##
==========================================
+ Coverage   80.33%   80.94%   +0.60%     
==========================================
  Files        4780     4780              
  Lines      201867   202196     +329     
  Branches    11503    11503              
==========================================
+ Hits       162174   163669    +1495     
+ Misses      39438    38272    -1166     
  Partials      255      255              

Impacted Files	Coverage Δ
src/sentry/conf/server.py	93.55% <85.71%> (-0.23%)	⬇️
src/sentry/integrations/jira/views/base.py	100.00% <100.00%> (ø)

... and 153 files with indirect coverage changes