refs: https://github.com/TryGhost/Ghost/issues/11407
- there's no need for us to process the whole document as it's only used for excerpts
- emails need the whole document, but they're managed seprately now
- this is more efficient and means we could potentially do it on the fly 🤔
refs: https://github.com/TryGhost/Team/issues/1609
- Split html-to-plaintext into a function for excerpts and emails
- Updated all usages so they use the correct function
- There's currently no difference between the two