Add rating mixin

[JonnyWong16]

Description

Adds RatingMixin to all objects which can have user star ratings.

Breaking change:

• Keyword parameter changed from rate(rate) to rate(rating).

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable