Implement conditional workflow steps

[mvdbeek]

This PR implements executing tool and subworkflow steps conditionally, based on the result of a javascript expression. The workflow editor will for now only allow the simplest expression, which is a connection to a boolean parameter (from an expression tool or a workflow input).
An example gxformat2 workflow looks like this:

class: GalaxyWorkflow
inputs:
  should_run:
    type: boolean
  some_file:
    type: data
steps:
  cat1:
    tool_id: cat1
    in:
      input1: some_file
      should_run: should_run
    when: $(inputs.should_run)

If when evaluates to true the step will run, if it evaluates to false it will not run.
This is also implemented for mapped over tool and workflow steps; if an output is skipped it will be produce a null value. You can think of this as an optional output. That means you can use the pickValue expression tool to pick for instance the first created output for two mutually exclusive processing steps.

Includes a whole new infrastructure for displaying scheduling warnings, failures and cancellations:

Remaining work:

• Update the workflow editor
• Some visual indicator for partial collections in the history ?

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]

I think the backend is ready for a review ... I would probably split out the UI work into another PR (I should wrap up the first part of the workflow editor overhaul, that would make this easier).

[jmchilton]

This is so complicated and we're only basically testing boolean parameters right?

What if an integer is sent - what happens? A dataset? Etc...

Can we have a test that the workflow dies if an invalid expression is set in there?

[mvdbeek]

I've included a bunch of tests and I've also added "invocation messages", see the screenshot in the description ... I think we're now displaying (almost?) all of the possible workflow errors in the API and UI.

[mvdbeek]

Test errors are unrelated

[jmchilton]

This is nothing right? How did we let this in 😆. It seems it hasn't caused any harm yet!