Why this change?
This change adds validation for the default value for `type: objects` theme
settings when a setting theme field is uploaded. This helps the theme
author to ensure that the objects which they specifc in the default
value adhere to the schema which they have declared.
When an error is encountered in one of the objects, the error
message will look something like:
`"The property at JSON Pointer '/0/title' must be at least 5 characters
long."`
We use a JSON Pointer to reference the property in the object which is
something most json-schema validator uses as well.
What does this change do?
1. This commit once again changes the shape of hash returned by
`ThemeSettingsObjectValidator.validate`. Instead of using the
property name as the key previously, we have decided to avoid
multiple levels of nesting and instead use a JSON Pointer as the key
which helps to simplify the implementation.
2 Introduces `ThemeSettingsObjectValidator.validate_objects` which
returns an array of validation error messages for all the objects
passed to the method.
