Refactor app config flow (incl. dealing with flask env deprecation)

[GustaafL]

Description

Refactor the flow in which the app config is loaded. This solves the issue where the documentation was unable to build by using a separate flow/function for the config loading for documentation, testing and (production, development, staging)

How to test

Execute the tests, try to build the documentation, starting flexmeasures with the environment variables coming from different locations.

[nhoening]

Please, I need a description what this PR is doing. That's usually in the PR description.
There are two env settings handled now, and some refactoring.
What is the big picture?

[Flix6x]

We discussed splitting this PR into two:

1. A separate PR now contains the fix that lets our documentation build pass again.
2. The remainder of this PR would then be about refactoring how different environments are being set up in app.py, with the goal of creating more clarity (e.g. fewer if/else statements).

@GustaafL thanks for editing the PR description. Please also remove any subsections that just contain contents from the PR template, as they only distract.

[Flix6x]

I didn't really review this, but I just noticed these two things:

1. See comment in check_app_env.
2. It also looks like on of the checks is failing: https://github.com/FlexMeasures/flexmeasures/actions/runs/7745435994/job/21121419518?pr=969

[Flix6x]

The following print statement would need to be updated correspondingly, I suppose:

        print(
            f'Flexmeasures environment needs to be either "documentation", "development", "testing", "staging" or "production". It currently is "{env}".'
        )

[Flix6x]

@nhoening want to take over this PR, seeing as you authored most of our app config logic?

[nhoening]

This issue has as motivation "the issue where the documentation was unable to build".

That issue seems fine, looking at https://readthedocs.org/projects/flexmeasures/builds/

So I assume the motivation is now a refactoring for cleaner code. I'll have to review if I can wrap my head around this one of these days.