Why this change?
This change supports a property of `type: category` in the schema that
is declared for a theme setting object. Example:
```
sections:
type: objects
schema:
name: section
properties:
category_property:
type: category
```
The value of a property declared as `type: category` will have to be a
valid id of a row in the `categories` table.
What does this change do?
Adds a property value validation step for `type: category`. Care has
been taken to ensure that we do not spam the database with a ton of
requests if there are alot of category typed properties. This is done by
walking through the entire object and collecting all the values for
properties typed category. After which, a single database query is
executed to validate which values are valid.
cac60a2c6b