o-toggle complete breaking checklist

[notlee]

Origami plan to release major new versions of low level components, which remove technical debt from recent additions such as component branding, and implement more recent proposals such as adding a primary Sass mixin and removing custom classes from many Sass interfaces.

steps

• Add MIGRATION.md
• Add a primary mixin, removing other mixins where possible.
• If a mixin is required which does not output a selector, e.g. for component composition, create an oComponentContent mixin with options like the primary mixin (Content infers the mixin does not output selectors).
• Replace methods to customise a component's style globally with a mixin oComponentCustomise e.g. so a whitelabel user can customise the style of a primary o-buttons button.
• Replace mixins which add new variations of a component with an oComponentAddSomething mixin e.g. so a user may output a new o-buttons button, alongside primary buttons etc.
• Remove custom classnames.
• Use the $system-code variable for the source param in image service requests, error if not found, update obt to ^ 8.2.2 locally and in circleci config.
• Remove existing semver ranges e.g. for o-brand and o-icons "o-brand": ">=2.2.0 <4",. We can't add new features to components used with a semver range without back-porting.
• Use esm consistently over cjs module syntax.
• Solve existing issues labeled "breaking" if possible.

[notlee]

Closing this to keep track of progress. The PR can be found here: #27. It will be released as part of the major cascade.