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)

plume-models/src/db_conn.rs

@@ -0,0 +1,37 @@
+use diesel::{
+    pg::PgConnection,
+    r2d2::{ConnectionManager, Pool, PooledConnection}
+};
+use rocket::{Request, State, Outcome, http::Status, request::{self, FromRequest}};
+use std::ops::Deref;
+
+pub type PgPool = Pool<ConnectionManager<PgConnection>>;
+
+// From rocket documentation
+
+// Connection request guard type: a wrapper around an r2d2 pooled connection.
+pub struct DbConn(pub PooledConnection<ConnectionManager<PgConnection>>);
+
+/// Attempts to retrieve a single connection from the managed database pool. If
+/// no pool is currently managed, fails with an `InternalServerError` status. If
+/// no connections are available, fails with a `ServiceUnavailable` status.
+impl<'a, 'r> FromRequest<'a, 'r> for DbConn {
+    type Error = ();
+
+    fn from_request(request: &'a Request<'r>) -> request::Outcome<Self, Self::Error> {
+        let pool = request.guard::<State<PgPool>>()?;
+        match pool.get() {
+            Ok(conn) => Outcome::Success(DbConn(conn)),
+            Err(_) => Outcome::Failure((Status::ServiceUnavailable, ()))
+        }
+    }
+}
+
+// For the convenience of using an &DbConn as an &PgConnection.
+impl Deref for DbConn {
+    type Target = PgConnection;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}