Students losing work when they don't save annotations before leaving the page (L)

**Feature request**
Some users would like to see annotations auto-saved as they write them. This way even if the annotations don't get posted before the tab closes they are recoverable.

In one use case a user accidentally closed a tab with an open (and lengthy) annotation they had been writing for some time. It's certainly possible that users will accidentally slip up and close a tab when annotations are as yet un-posted.

Users would likely also use this feature to draft annotations. We definitely see students in the LMS app who insist on drafting multiple annotations before posting them and these students occasionally accidentally close the assignment tab before posting. We do tell professors to encourage students to Post and then edit annotations later if needed, but that usually only comes up after there’s been at least one mistake and lost annotations.

From this Slack convo: https://hypothes-is.slack.com/archives/C2BLQDKHA/p1616380824007600

No files, tests, or entry points are identified in the issue. First clarify how drafts should be stored and recovered, when they should be saved, and what happens across tabs or sessions; done means an unfinished annotation survives leaving the page and can be recovered without losing posted content.

Written by the indexing model from the issue text.