Big repository reorganization

The code is divided in three crates:
- plume-common, for the ActivityPub module, and some common utils
- plume-models, for the models and database-related code
- plume, the app itself

This new organization will allow to test it more easily, but also to create other tools that only reuse a little part of
the code (for instance a Wordpress import tool, that would just use the plume-models crate)

src/routes/errors.rs

@@ -1,7 +1,7 @@
 use rocket_contrib::Template;
 use rocket::Request;
 use rocket::request::FromRequest;
-use models::users::User;
+use plume_models::users::User;
 
 #[catch(404)]
 fn not_found(req: &Request) -> Template {