Rationalize styling

[jennybc]

Various styling issues have accumulated:

Are we consistent in our use of ui_THIS() vs ui_THAT()? For example, I think URLs should probably be treated with ui_field() not ui_value(). Maybe we should create ui_url() (even if it's aliased to ui_field().

The way we handle multi-line messages is all over the place. Mostly, I think, because it's not clear what you're actually supposed to do. Write that down in principles.md then fix obvious violations.

[krlmlr]

I agree each kind of output should have its own ui_...() function, even if it's only a redirect. Specialized ui_...() functions could check the input format and warn (once) if there's a mismatch.

[jennybc]

Roll this into #956. Converting to cli would be an obvious time to rationalize usage and styling of ui_*() functions.