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/session.rs

@@ -6,8 +6,10 @@ use rocket::{
 };
 use rocket_contrib::Template;
 
-use db_conn::DbConn;
-use models::users::{User, AUTH_COOKIE};
+use plume_models::{
+    db_conn::DbConn,
+    users::{User, AUTH_COOKIE}
+};
 
 #[get("/login")]
 fn new(user: Option<User>) -> Template {