Save draft content locally when authoring posts or comments

## Issue Description

We should store post and comment content in localStorage while composing posts or responses, so that a browser crash, navigation mistake, or other issue does not cause drafted content to be lost.

This issue is split from #459 which made it more obvious when work _would_ be lost, but provides no recovery option.

## Steps to reproduce:

Sign into a H2 eg https://updates.hmn.md/, write a comment on a P2 post, click on the post name of a different post. You will be taken to the individual post page. Click back to go back to where you were, click on "reply" button.
Alternatively, begin to compose a post, then navigate to a different site. Return to H2 and click the new post button again.

**Expected Result:** Your draft content is still present in the text field. Possibly it highlights that it is an autosaved version of the comment.

**Actual result:** The field is empty and your content has gone forever into the great Cloud.