Allow more control over allowing includes

[lgebhardt]

Deprecates and replaces the allow_include config option

Replaces allow_includes with default_allow_include_to_one and default_allow_include_to_many configuration options.
Adds support for allow_include option on Relationships to override the config settings.

Purpose:

• Includes without pagination allows potentially resource intensive requests.
• Pagination of includes is a non-trivial problem to solve efficiently with SQL databases
• By allowing includes defaults to be set for to_one and to_many relationship types separately includes for the riskier to_many relationships can be disabled by default, while still allowing them on specific relationships if desired.

[joegaudet]

This is awesome, would be cool to have the context passed in, similarly to fetchable_fields useful to remove some depending on the current user.

[lgebhardt]

@joegaudet Good idea

[lgebhardt]

Updated for @joegaudet so you can now provide a boolean, a callable name, or a lambda for allow_include. Context is passed to the callable and lambda.

[dgeb]

Looks pretty good. Needs one simple fix. I'd also feel more comfortable if the recursion logic in check_include were tested.