Reactify custom WTForms fields

[duartegalvao]

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This PR adds the following components:

• a FinalPersonLinkField, so that the PersonLinkField can be added to final forms
  • also includes a wrapper for abstracts and contributions, with predefined roles
• a SessionColorPicker and a FinalSessionColorPicker
• a LocationField and a FinalLocationField
• an ItemListField, a FinalItemListField and a FinalReferencesField

Moreover, it includes a new dev module with a page to test all of the custom React fields in /dev/react/fields. Also, the ChoicesSetup was refactored using the new ItemListField.

This will be important for future rectifying of timetable-related dialogs for #6220.

[ThiefMaster]

I recommend adding some wrappers for the different object types - check the PersonLinkListFieldBase subclasses in Python what roles we set and on which conditions this depends.

[duartegalvao]

I recommend adding some wrappers for the different object types - check the PersonLinkListFieldBase subclasses in Python what roles we set and on which conditions this depends.

I guess we can skip the abstracts one for now, right?

[ThiefMaster]

yeah... unless we want to move all dialogs in one go, which might actually not be such a bad idea considering the huge amount of awful complexity the wtforms-based backend of this field requires (for sending back the data when re-rendering the form)

[duartegalvao]

merged into timetable branch