Headings should include id's so they can be linked to

[zspencer]

In order to deep-link into particular sections of long-form pages
As a website reader
I would like headings to include ids (and maybe even be links to themselves?)

Notes

There appears to be significant dissent around whether or not auto-generating headings is safe, but because we are not rendering user-generated content; I think it's safe for us to use one of these plugins and configure our markdown renderer appropriately