Replace all http code in epi/eframe/egui_glium/egui_web with ehttp (#697)

I've extracted all the http request code and turned it
into its own crate at <https://github.com/emilk/ehttp>.

There was never a reason for the HTTP request library to be part of
`eframe`. Much better to have it as its own crate!

emath/Cargo.toml

@@ -17,6 +17,9 @@ include = [
   "Cargo.toml",
 ]
 
+[package.metadata.docs.rs]
+all-features = true
+
 [lib]
 
 [dependencies]