Correctly handle [] and {} as invalid inputs to BooleanField

Add note about field level validation, fixes #3306

The documentation added warns the user that field level validation will
be skipped if the field is declared on the model with the parameter
`required=False`.

Fixed typo in ScopedRateThrottle example views

Improve documentation of default argument for fields

Don't pass `allow_empty` to `ListSerializer`'s children.

Helps with backward compatibility.

Update 6-viewsets-and-routers.md

The `set_context` method there is the same as for the default field.

DurationField accepts integers

The documentation now not only mentions that callables are acceptable
arguments, it also tells the reader what the callable should look like.

Replacing self.tag with self.tag_name in Generic Relationships documentation.

Added django-url-filter to 3rd party libraries

Fix typo

Don't error on missing `search_fields` attribute.

Resolve empty HTML charfield behavior.