Draft: Fix HTML saving / `transform_to_wikitext` 412 errors

See also https://phabricator.wikimedia.org/T354037

While this lead to spurious errors before it stopped working completely recently.

We have multiple ways to fix this:

1. don't use the if-match header
2. always retrieve HTML with the undocumented stash=true parameter
3. retrieve again with stash=true on transform

Instead of #2 and #3 we could let the user decide if they intent to save when getting the HTML and only stash in this case and only re-retrieve with stash on transform if necessary.