Add features x-head header style

[notlee]

This issue defines class names for the new header to align with the work of spark and storytelling teams, see Slack and Jira.

• [ ] Add the features x-head heading style to o-editorial-typography
  • font: display, scale: 4, weight: bold
  • mixin oEditorialLayoutHeadingFeaturesXhead
  • class .o-editorial-typography-features-xhead
• [ ] Match the new heading spacing with h3/4 headings in o-editorial-layout
• [ ] Backport changes to old versions of components (for deprecated Bower support)
• [ ] Work with customer products design to update the Figma UI kit

Relates to: https://github.com/Financial-Times/origami/issues/190

[notlee]

Marked as blocked whilst we wait for timelines from other teams. It won't take long for us to do this work, so let's do it as other teams pick up their part, to avoid making changes later or adding something which isn't used.

[JakeChampion]

This is currently blocked on the work to enable new subheading options in Spark -- https://financialtimes.atlassian.net/browse/SP-1196