[RFR] Improve form look and feel

[fzaninotto]

Material design no longer advocates the usage of fields denoted by a simple underline. Only the "filled" and "outline" variants are documented in the material design guide.

The major bump in react-admin v3 is the right occasion to reflect that change. But we should let an escape hatch for users who absolutely don't want filled inputs. That is possible with the material ui theme, and we should make it possible to override the variant at the form level.

Also, I want to make forms more compact, so we use a dense margin by default.

Before	After

• Provide a default theme with less ugly filled variant than mui default light
• Provide a default theme with less ugly filled variant than mui default dark
• Allow users to override margin and variant at the form level
• Make common inputs dense and filled by default
• Make other inputs dense and filled by default
• Update Form documentation to mention the new props
• Add upgrade guide

[fzaninotto]

Switching to RFR. The failling tests also fail on next.

[fzaninotto]

Thanks for searching and fixing that hard bug!