Added local revisions to the editor as backstop against data loss

[cmraible]

ref https://app.incident.io/ghost/incidents/107

• We have a rare bug that causes the initial POST request to create a new post from the editor to be skipped or fail. Subsequent PUT requests then fail because there is no post ID, potentially resulting in data loss. The aim of this commit is to start saving revisions of posts in the editor to the browser's localStorage, as a last-ditch option to restore lost work.
• Since we don't know where the bug is yet, and to protect against future bugs, we've deliberately avoided depending too heavily on the lexical-editor controller or the ember store. We've aimed to create a direct route to the state in the editor, by hooking into the updateScratch method (effectively the onChange handler for the editor).
• The scheduleSave function on the new local-revisions service is called immediately upon any changes to the state of the lexical editor, which is effectively every keystroke. The service has some logic and timeouts, so it doesn't actually save a revision on every change to the editor.
• The "schema" of the datastore is a simple key-value store, where the key is of the format: post-revision-${postId}-${timestamp} if the post has an ID, or post-revision-draft-${timestamp} for an unsaved draft. There is also an array of all the revisions' keys, which allows us to clear all the revisions without having to loop over every key in localStorage (along with some other conveniences, like filtering).
• There is currently no UI for viewing/restoring revisions. In the event that you need to restore a revision, you can access the service in the browser console. You can access all the saved revisions using the list() method, which logs all the revisions to the console by title & timestamp. You can then choose a revision to restore, and call restore(revision_key), which will POST the revision's data to the server to create a new post.
• Since localStorage data is limited to a 5mb quota in most browsers, the service has a mechanism for evicting the oldest revisions once it meets the quota. If a save fails because it would exceed the quota, the performSave method will evict the oldest revision, then recursively try to save again.