[WIP] [Widget Params] Restyled source editing (3 of 3)

[ranbena]

Code work is done, but labeled WIP to allow feedback first on the user experience.
Play with it in https://deploy-preview-3377--redash-preview.netlify.com.

What changed and why?

• Source type dropdown changed to radio buttons. I think it's extremely valuable for clarity to have all options laid out instead of hidden inside the dropdown.
• When no existing params, instead of hiding the option, it is disabled and has info available.
• "OK" button disables on form error.
• Source type labels are more coherent. @susodapop lmk if you have any suggestions here.
• If current keyword not in existing params, don't show it as an option. This was possible before but makes no sense as it essentially means adding a new dashboard param.

Before:

After:

[ranbena]

Moved to #3332