Improve API routing

[Twixes]

Changes

Our API endpoints in this form: /api/events have confusingly been not working… the difference is just the lack of a trailing slash. That has been a longstanding issue, hard to diagnose for users and annoying when developing. This PR fixes that, along with lacking support for personal API keys in /decide (superseded by #1553), and also adds API-specific 404 handling (as to not confusingly 302 to the app login page when hitting a wrong endpoint).
Updated URLs in a few tests to make this covered by tests.

Checklist

• Backend tests (if this PR affects the backend)

[yakkomajuri]

@Twixes maybe take a look at #1553 to see the approach you'd wanna have to the /decide endpoint since I set up the same authentication mechanism on /decide as /capture

[yakkomajuri]

@Twixes when you get a chance take a look at the conflicts and then I could pick up the review on this, unless there's any objection to that

[yakkomajuri]

This looks good to me. I made some changes to also make the internal use endpoints flexible, as well as made sure the tests were testing endpoints both with and without a trailing slash. Added comments so that people can make sense of the "inconsistency".

I made changes regarding endpoints that weren't in the scope of this PR but that I thought made sense. @Twixes see if the changes make sense and otherwise this should be good to go.

[Twixes]

Makes sense. 👍

[Twixes]

Will have to take a look at these Cypress tests finally… But this should be OK for the moment.

[yakkomajuri]

Yeah I was thinking of doing that and then we got the debug bar PR merged, all new PRs pass, but old PRs that get merged with master don't?