Port the paragraph types shared by all three sites into the core

Problem/Motivation

Three paragraph types appear on every site — an FAQ accordion, a reference to a global block, and a raw HTML block — and another three appear on two of them. They are implemented once per site, and their bugs are fixed one site at a time. They are also the types a new site is most likely to have, so they are what makes the module useful out of the box. The rest do not generalise: of the 81 types across the sites, 73 exist on exactly one.

Proposed resolution

• Compare the implementations of each shared type, decide the correct behaviour, and record where they disagreed.
• Ship default structures for them, applied when a site has a paragraph type whose fields match — as a suggestion in the queue, never silently.
• Cover each with a kernel test on the fixture model.

Remaining tasks

Everything.