Linting / validation for User Task when no form selected

[harmeet-status]

Add validation for User Task when form is not selected. Show icon showing that the task is not executable, but allow saving the process model. For Level 1 & 2 process design, allow hiding of validation failure icon.

In the BPMN Editor:

• Display validation errors in a different view component that can be easily shown and hidden
• Validate that a user task has a form connected to it.
• Maybe validate a few other easy to check things (DMN exists?, Call Activity hooked up?)
• Script Unit Tests (run em, and see if something breaks) - show as any other error
• BPMN Unit Tests
• Check for a good BPMN Checker / Linter (https://github.com/camunda/linting)

Another linting related issue where there is a diagram that is invalid but we never let the user know: https://github.com/sartography/spiff-arena/issues/492

[madhurrya]

@calexh-sar please check this