refactor(web/converse): Refactor first handlers to rouille

This commit starts the refactoring process towards dropping actix (and
tokio, ...). It builds, but at this commit, Converse does *not* work.

I decided to commit to avoid more ridiculous diffs.

Included changes:

* Added dependency on rouille.

* Refactored DbExecutor (formerly actix actor) to simply be a type
  with a few methods. Most actor messages still exist as they are
  being referred to by handlers.

* Started refactoring two of the handlers (and their related renderer
  functions) into Rouille's call scheme.

Important note: Rouille does not have safe session management out of
the box, and it will need to be implemented as this progresses.

Change-Id: I3e3f203e0705e561e1a3392e8f75dbe273d5fa81
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2861
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

web/converse/src/main.rs

@@ -30,6 +30,7 @@ extern crate log;
 #[macro_use]
 extern crate serde_derive;
 
+extern crate rouille;
 extern crate actix;
 extern crate actix_web;
 extern crate chrono;