Form improvements for combination repository field

[agjohnson]

• Requires Form improvements to project repository fields readthedocs.org#12654

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• Give guidance when user doesn't have a connected account
• Give guidance when user doesn't own remote repository connection
• Handle differences in form error state and user state better, like don't reset remote_repository field when form field errors or when loading default state
• Remove hardcoding of some of the field states

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Screenshots:

[humitos]

I haven't took a look at the code yet, but screenshots look great!