Make it possible to plug in context processors included with apps

[hmpf]

Bonus: halt setup with a traceback if the JSON in EXTRA_APPS/OVERRIDING_APPS is buggy!

How to use: add a context processor in an app (for instance argus_htmx.context_processors in Uninett/argus-htmx-frontend#15) to the app, it's a list on the key context_processors.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 84.81%. Comparing base (05d26f0) to head (9187b71). Report is 10 commits behind head on master.

Files	Patch %	Lines
src/argus/site/utils.py	94.11%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #810 +/- ## ========================================== + Coverage 84.50% 84.81% +0.30% ========================================== Files 76 76 Lines 3808 3832 +24 ========================================== + Hits 3218 3250 +32 + Misses 590 582 -8 ```

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[github-actions[bot]]

Test results

       7 files     616 suites   22m 55s :stopwatch:    488 tests    487 :heavy_check_mark: 1 :zzz: 0 :x: 3 416 runs  3 409 :heavy_check_mark: 7 :zzz: 0 :x:

Results for commit 9187b715.

:recycle: This comment has been updated with latest results.

[hmpf]

Can't see anything wrong with the added functionality itself (except it seems to me that GNOC will still bypass all these hook-ins by writing their own settings file by hand?)

I've grown fond of it myself, much quicker adjusting settings by changing a json file that I cat(1) into the environment variable than adjusting the settings file and urls.py when flipping between branches with different needs. Pretty good dev UX!

[sonarcloud[bot]]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud