This adds support for editing polymorphic fields. Props go to @pedantic-git, on whose work this is based. This PR supersedes https://github.com/thoughtbot/administrate/pull/984.
An important aspect of this PR is how it handles the polymorphic value. At heart, polymorphic values are split in two: an id and a type. Using the globalid gem we can obtain a string that represents both. However, we still need to tell that this string is indeed a globalid and not a literal value when we get to Administrate::ApplicationController#resource_params. I handle this by splitting the field in two:
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my_field[value]: the global id, which includes both type and instance id. -
my_field[type]: literally"Administrate::Field::Polymorphic"
Then #resource_params can look at the param values and treat them appropriately. If it's a simple value such a string, it does nothing special. However, if it comes in this value/type form, we treat it as polymorphic, reading the global id.