Add fallback when API call fails.

[benmuth]

This PR adds a fallback in case the user is running an ArchiveBox version without a REST API (pre v0.8.0).

[pirate]

Excellent

Can you help record these three ~5-10sec screenrecording gifs, we'll use them for the new docs:

1. setting up the server in the config UI + showing it turn green showing server is valid
2. submitting a URL by clicking the menubar action button from a tab
3. refreshing the server admin UI and showing the URL show up in the snapshot list

I like this app, but there are many others too that can make GIFs easily: https://gifox.app/

[benmuth]

@pirate For people using 0.7.3, how should we handle auth? Should we add fields to the config UI to let them set the username and password, just like with the API key?

[pirate]

You should be able to just make a request to /add with the browser's existing cookies, you can see how the current codebase does it on the main branch.