ui: add initial Profile hover previews

The idea with these is that on notedeck you can just hover your cursor
over a profile link to see the profile. I just have a stub for now, but
full design coming soon after.

Also simplify the preview system even further with a macro. In the
future I imagine we can grep every preview in the codebase, and then
include this as a string inside this macro. This is some kind of
template metaprogramming insanity but in theory it could work.

Signed-off-by: William Casarin <jb55@jb55.com>

src/ui/profile/mod.rs

@@ -0,0 +1,3 @@
+pub mod preview;
+
+pub use preview::ProfilePreview;