Implemented Alternative Checkboxes Style Settings Toggle

[damiankorcz]

📕 Description

This implements the feature mentioned in this issue: #255

• I've implemented the toggle feature in Style Settings.
• By default, your Alternative Checkbox styling is applied and the toggle can disable it.

Once toggled to disable, the Alternative Checkboxes are reset to default styling:

🔖 Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

✅ Checklist

All checkboxes below must be followed and checked.

• I followed the Developer's Guide when building for Primary

Primary uses Sass and Grunt so building the project is important to ensure the final production code is up to date with changes from all the .scss and .css files. If you are unsure about this please look into the Developer's section inside the README or if you have any questions, post an issue or send us a message before sending in this Pull Request.

• My code follows the style guidelines of Primary
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings