Replace local_settings.py with .env

[ddabble]

This depends on #766.

Proposed changes

Using environment variables is a more standard way of configuring settings.

Also removed the NOTE: This must be changed in production! comments in settings.py, as env.py now enforces that those settings have been set.

Review guidance

Create an empty .env file directly inside the repository folder, fill it by copying the contents of .env.example, and try running the project.

Deployment notes

Must create a .env file on the prod server with the corresponding values in the existing local_settings.py (and local_settings_post.py) file.

Checklist

(If any of the points are not relevant, mark them as checked, so that it's easy to see which points you've handled or not)

• I've run makemigrations, makemessages and compilemessages
• I've written tests that fail without these changes (if relevant/possible)
• I've manually tested the website UI with different device layouts
  • Most common is to test with typical screen sizes for mobile (320-425 px), tablet (768 px) and desktop (1024+ px), which can easily be done with your browser's dev tools
• I've manually tested with different users locally
  • This can be e.g. anonymous users (i.e. not being logged in), "normal" non-member users, members of different committees, and superusers
• I've made sure that my code conforms to the code style guides
  • It's not intended that you read through this whole document, but that you get yourself an overview over its contents, and that you use it as a reference guide / checklist while taking a second look at your code before opening a pull request
• I've attempted to minimize the number of common code smells
  • See the comment for the previous checkbox
• I've added documentation
  • E.g. comments, docstrings, or in the README
• I've added my changes to the "Unreleased" section of the changelog, together with a link to this PR
  • Mainly the changes that are of particular interest to users and/or developers, if any
• I've added a "Deployment notes" section above and labelled the PR with has-deployment-notes
  • ...if anything out of the ordinary should be done when deploying these changes to the server (e.g. adding/removing an environment variable, manually creating/changing some objects, running a management command, etc.)
• I've structured my commits reasonably
  • Some tips on this can be found in "Write Better Commits, Build Better Projects" and in the code style guides
  • Restructuring commits after they've already been committed, can be done using e.g. interactive rebase; see the Git docs on rewriting history

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.04%. Comparing base (38c1bcf) to head (75702d9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #767   +/-   ##
=======================================
  Coverage   88.04%   88.04%           
=======================================
  Files         153      153           
  Lines        6224     6224           
=======================================
  Hits         5480     5480           
  Misses        744      744           

Files with missing lines	Coverage Δ
src/util/url_utils.py	95.65% <100.00%> (ø)
src/web/admin_urls.py	93.33% <100.00%> (ø)
src/web/urls.py	95.34% <100.00%> (ø)

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[TheStrgamer]

This works as intended when i run it locally.
The changes in the code also look good.