Implement conditional workflow editor UI, optional connections, track and display invalid connections

[mvdbeek]

This adds editor support for conditional workflow steps:

This also changes how we deal with invalid connections (like when you make a step conditional, or change a steps's output datatype, or when the tool is not installed ...)
This makes it so much easier to replace invalid connections without missing something:

Fixes #13527.

It'd also be the basis for transitively changing e.g the input collection type ... if we can just show that a connection is now invalid we can let the user evaluate if they really want to do it.

How to test the changes?

(Select all options that apply)

• 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 and all my past contributions to the core galaxy codebase under the MIT license.

[mvdbeek]

That one selenium test error is unrelated

[dannon]

This is fantastic! We've discussed a few iterations of improvements out of band but this is a great addition as-is, let's get it in.