[2.x] Add support for terms of service and privacy policy checkbox

[themsaid]

This PR adds support for a Feature::termsAndPolicy feature that shows a checkbox on registration that users need to check before they can continue.

It also publishes a terms.md and policy.md files in the project root where people can write the content in markdown and it'll be parsed by Parsedown and formatted by Tailwind's prose plugin.

[ConnorHowell]

One question, should route registration not check for 'Jetstream::hasTermsAndPrivacyPolicyFeature()' first?

This way it's inline with the other optional features that register routes.

[taylorotwell]

One question, should route registration not check for 'Jetstream::hasTermsAndPrivacyPolicyFeature()' first?

This way it's inline with the other optional features that register routes.

Fixed this.