Add possibility to change base URL / collection URLs

[rfc2822]

See #130

[emteelb]

Another use case where it would be convenient to be able to edit the URL of an existing account: URL is a LAN address and port number. When traveling, I want to be able to quickly change this to a dynamic IP service name and a port forward number.

[unusualevent]

I forgot that this still existed as a bug. I swapped to tailscale, and could not get .local addresses to work for my phone. Every other app (like syncthing) could be easily edited. (I don't know where in the code I would get started contributing to an edit button)

[unusualevent]

or... at the very least, to display the old one.