Add various HTTP headers to improve security

[Andrew-Dickinson]

Improve our security by following the HTTP header recommendations from the following resources:

• https://realpython.com/django-nginx-gunicorn/#incorporating-nginx
• https://securityheaders.com/?q=db.nycmesh.net%2Fadmin&followRedirects=on

Setting X-Forwarded-Proto also fixes a bug which caused a mixed content error on the changes from #636, since it fixes the value Django uses for request.scheme (currently set to http in prod due to the reverse proxy)

[codecov[bot]]

Codecov Report

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

Project coverage is 94.18%. Comparing base (28649c1) to head (c08c817). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #643 +/- ## ======================================= Coverage 94.18% 94.18% ======================================= Files 80 80 Lines 3251 3251 ======================================= Hits 3062 3062 Misses 189 189 ```

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