Adds new Gdoc type: linear-topic-page

[ikesau]

Supersedes #2868

1. Adds a new type to the gdoc type enum that we can use to add custom behaviours for linear topic pages.
2. Increases the size of headings in linear topic pages to approximate the same hierarchy that they currently have with their super headings.
3. Adds logic to wrap headings that were originally h1's (i.e. the grey bar section headings) with horizontal rules.