Allow for custom components in the body text of aticles

[cuibonobo]

Example: https://fjord.style/going-into-depth

I've seen this done with a Markdown + React library. Another way to solve this would be with block-style editing reminiscent of a Jupyter notebook. An issue I have with the blocks solution is that the blocks don't have a semantic relationship between each other. For Markdown the semantics aren't explicit either though... For example:

# An H1 heading
Some text that belongs to the H1 heading.

## An H2 heading
Some text that belongs to the H2 heading.

---
My _intention_ with this paragraph is to close the H2 topic and return to the H1 topic, but I don't think that's actually possible without enclosing tags of some sort.

The UI for transcribing intended semantic meaning is probably outside the scope of this issue, though.