Proper cache control

[artoonie]

From @skaphan:
Add updated_at field to JsonConfig model to track modification times
Use ConditionalGetMixin for all visualization views
Short-circuit 304 responses in VisualizeEmbedded before expensive computation

Local PR created to enable Heroku CI to run

[artoonie]

It looks like this is failing several of the caching tests:

FAIL: test_movie_task_without_celery (movie.tests.MovieCreationTestsMocked.test_movie_task_without_celery)
Test that the movie gets created when calling create_movie_task() directly
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/app/.heroku/python/lib/python3.13/site-packages/mock/mock.py", line 1468, in patched
    return func(*newargs, **newkeywargs)
  File "/app/movie/tests.py", line 95, in test_movie_task_without_celery
    self.assertEqual(TextToSpeechCachedFile.objects.count(), 4)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 1 != 4
======================================================================
FAIL: test_cache_speed (visualizer.tests.testLiveBrowserHeadless.LiveBrowserHeadlessTests.test_cache_speed)
Tests that caching works and that second loads are faster,
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/app/visualizer/tests/testLiveBrowserHeadless.py", line 261, in test_cache_speed
    is_cache_much_faster(baseUrl, True)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/app/visualizer/tests/testLiveBrowserHeadless.py", line 247, in is_cache_much_faster
    self.assertGreater(avgLoadTimeWithoutCache, avgLoadTimeWithCache * 2)
    ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 60.666666666666664 not greater than 119.33333333333333
======================================================================
FAIL: test_cache_works (visualizer.tests.testLiveBrowserHeadless.LiveBrowserHeadlessTests.test_cache_works)
Tests that caching doesn't call the heavy graph-generation function more than once
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/app/.heroku/python/lib/python3.13/site-packages/mock/mock.py", line 1468, in patched
    return func(*newargs, **newkeywargs)
  File "/app/visualizer/tests/testLiveBrowserHeadless.py", line 289, in test_cache_works
    self.assertEqual(count_cache_misses_mocked(url), 0)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 1 != 0
----------------------------------------------------------------------
Ran 52 tests in 287.892s
FAILED (failures=3)

As well as some linter issues:

-----> Running test command `./scripts/run-tests.sh`...
Compressing... done
Compressed 22 block(s) from 26 template(s) for 1 context(s).
Checking for pylint issues
************* Module visualizer.views
visualizer/views.py:174:4: C0116: Missing function or method docstring (missing-function-docstring)
visualizer/views.py:182:12: C0103: Variable name "last_modified" doesn't conform to camelCase naming style (invalid-name)
visualizer/views.py:183:12: C0103: Variable name "if_modified_since" doesn't conform to camelCase naming style (invalid-name)
visualizer/views.py:185:16: C0103: Variable name "if_modified_since" doesn't conform to camelCase naming style (invalid-name)
visualizer/views.py:165:0: R0903: Too few public methods (1/2) (too-few-public-methods)
-----------------------------------
Your code has been rated at 9.99/10
These files must be perfectly linted
Checking for autopep8 differences
-----> test command `./scripts/run-tests.sh` failed with exit status 1