Frontend regression testing

[fekoch]

This PR adds a subclass of LiveServerTest: VisualRegressionTestCase. This allows to write test cases like StaffSemesterViewRegressionTest, which are standard live server tests where you can use self.trigger_screenshot(<identifier>).
The screenshot is then compared to a known good screenshot and the testcase fails, if there is a large enough difference (as specified by us) between the two.

The CI needs to provide three environment variables for the test to establish a connection to testing api:

• VRT_APIURL
• VRT_APIKEY
• VRT_PROJECT

[niklasmohrin]

we also need to declare this dependency

[richardebeling]

Out of curiosity, roughly how painful would it be to do this without requests? If it's just 10-ish more lines of python with urllib, I'm tempted to use that instead

[niklasmohrin]

On the other hand, we have requests in our uv.lock either way through mozilla-django-oidc; I don't have a strong opinion towards either direction

[richardebeling]

Ah if we already install it anyway, fine with me

(although mozilla-django-oidc is a candidate for removal as soon as possible :D)

[fekoch]

The python standard library documentation explicitly recommends using the Requests package: https://docs.python.org/3/library/http.client.html

Considering that, I think it is fair to explicitly depend on it.