refs https://github.com/TryGhost/Team/issues/1206
- added `@allowBr` argument to `<KoenigBasicHtmlInput>` and used it in the callout card
- when enabled, adds `soft-break` atom and parser, enables `<br>` support in `cleanBasicHtml()`
refs https://github.com/TryGhost/Team/issues/931
- prototype button has served it's purpose in answering questions and is no longer being used
- groundwork is left in place for future component-based atoms
refs https://github.com/TryGhost/Team/issues/931
- adds handling for component atoms
- add prototype button atom to test atom behaviour
- add `Cmd+Shift+B` keyboard shortcut to create a dummy button atom when the `emailCardSegments` feature is enabled
refs https://github.com/TryGhost/Ghost/issues/9311
- pressing <kbd>Shift+Enter</kbd> will create a `soft-return` atom that adds a `<br>` element to the doc
- emulates many rich-text editors that have a similar functionality where it's desirable to add line breaks rather than starting new paragraphs
