Try to improve the interface layout of the API Backend form a bit.

More could be done to revamp this UI, but this is a quick stab at trying
to improve things with the current UI:

- Show the empty tables like the Backend Servers table with a message
  about adding a new backend. Hopefully this will make it less likely to
  skip over this, since previously this required section hinged on you
  clicking the floating "Add Server" button without much context. Now
  there's at least an empty table to maybe give you an idea that
  something needs to go there.
- Indent the fieldset form content under the legend header a bit. This
  hopefully makes the different distinct sections stand out a bit more,
  rather than this big form all running together.
- Increase margins around the fieldsets and use a different, taller font
  for the legend headers (this just seems to also help make the separate
  sections stand out a bit more).
Add a tip about recommending trailing slashes for URL prefix setup.
Add a bit of clarification text to the API host section.
Rudimentary validation to ensure 1 backend and url prefix is present.