o-icons complete breaking checklist

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

[x] Add MIGRATION.md
[x] Add a primary mixin, removing other mixins where possible.
[x] 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).
[x] 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.
[x] 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.
[x] Remove custom classnames.
[x] 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.
[x] 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.
[x] Use esm consistently over cjs module syntax.
[x] Solve existing issues labeled "breaking" if possible.
[x] Review README

Financial-Times / o-icons

o-icons complete breaking checklist #85