Support importing Readwise highlights

[jkomoros]

Now that there is embedding-based similarity, the more content in the card web, the more connections it can make.

I have thousands of highlights in Readwise, and it has an export function: https://readwise.io/api_deets

Card-web also has work and person cards, and could easily have quote cards. #465 .

It shouldn't be that hard to have a periodic import of Readwise quotes. And use the flags property to store things like the ID of the highlight.

The main problem with this is that the number of cards would become MASSIVE and almost certainly break some limit in the system.

Cards imported via this would have a flag.imported_from='readwise' that would allow by default not downloading them unless you hit a button to in the UI, since it might slow down the whole thing.

(Once we get to the point where some existing cards might not be downloaded, we need to overhaul the whole "a card that is not on client doesn't exist" treatment, and make it "if a viewer can view_unpublished hten we never pretend a card doesn't exist even if it's not on the client")