Add PriceClass variable so that it is configurable (V2)

[Tomdango]

Description

This includes the changes by @andylamp with a clean Git history. It introduces two new variables to change the price class of the CloudFront distribution, and to control the cache_control values of static assets.

See #20 for more detail.

Context

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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