Issue Forms render form preview and validate yaml before commit

[leonard84]

I really like the new form feature for the issues, but authoring the yaml files for that is unnecessarily cumbersome.
When you are editing a <form-name>.yml in the frontend, it already detects that you are editing a special issue form yaml and shows you some helpful documentation on the right hand side. However, if you click on the preview tab, you don't see a preview of the form, instead you just the the commit diff.
To actually see the form you have to submit it, and only then can you see the form and any yaml errors you made.
A pre-commit form preview and yaml validation would avoid useless commits, and make the whole experience much smoother.

[Kixunil]

Yeah, I had to do two fixup comits just when trying out the feature. Validation would be incredibly helpful.

[gavinr]

The diff isn't even showing for me:

Please support proper previews!

[Ridihush001]

Can't find where to state my issue

[melinath]

+1 - I can sort of get a preview after pushing the file to GitHub, but it doesn't actually look the same as the form looks after merge.