Allow modification of existing backends

[Daniel Tao]

Currently if you need to reconfigure a backend somehow, your only option is to delete it and re-add it. It should be easy to simply update it in place.

[Daniel Tao]

added the ability to modify existing backends (closes #2)

This involved quite a few changes:

1. adding “fields” to backend prototyps
2. removing backend snowflakiness so everything comes from config
3. adding a form in the sidebar to edit the current backend
4. adding a little * indicating when a backend has modifications

→ <<cset b3da3537a639>>