[WiP] Pipeline validation

[kzantow]

This is a catch-all clean up in preparation for a preview release and validation. Includes pipeline restyling that Brody mocked up.

TODO:

• hold off validations for new steps
• add a slow debounce for validation during field editing, remove blur listener
• check if the pipeline actually changed before running validation
• handle sheet stacking for nested steps
• moved nested step child editing to the step details view
• JENKINS-41361 add a blurb about how to use the load/save
• add a default pipeline
• fix deleting stages
• add empty child step placeholder for container steps

Blurb:

Empty child step placeholders:

[michaelneale]

tried this recently - looks 🐝 to me!

The validation is nice! We may need to massage the errors so it doesn't say "array shouldn't be 0" though but "a stage must have at least one step" (not sure if part of editor or validation code in declarative though? I think latter)

[kzantow]

@michaelneale yes, I'm going to add special handling for that. the message is coming from PMD validation. I also still need to tweak when the validation gets run, it's a bit too much right now

[michaelneale]

🐝 based on screenshots - will run it later, but worth a YOLO merge