Abstraction for fetching qualified app URL across webapp types

[jmchilton]

What did you do?

• Add abstraction for recovering URL.

Why did you make this change?

• https://github.com/galaxyproject/galaxy/pull/11608/files#r601625363

How to test the changes?

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these contributions under Galaxy's current license.
• I agree to allow the Galaxy committers to license these and all my past contributions to the core galaxy codebase under the MIT license. If this condition is an issue, uncheck and just let us know why with an e-mail to galaxy-committers@lists.galaxyproject.org.

[davelopez]

This definitely looks like the way to a proper solution! Thank you so much for taking the time to look into it. I was lost in the matter 😄

The url_for function is used in quite a lot of places all over the code base so, simply deprecating it, doesn't seem feasible. Also, I'm not sure if the FastAPI/Starlette version of url_for has the same signature/interface as the old one 🤔

[mvdbeek]

Very nice, thanks @jmchilton!