Use Ructe (#327)

All the template are now compiled at compile-time with the `ructe` crate.

I preferred to use it instead of askama because it allows more complex Rust expressions, where askama only supports a small subset of expressions and doesn't allow them everywhere (for instance, `{{ macro!() | filter }}` would result in a parsing error).

The diff is quite huge, but there is normally no changes in functionality.

Fixes #161 and unblocks #110 and #273

plume-models/src/lib.rs

@@ -1,8 +1,8 @@
 #![allow(proc_macro_derive_resolution_fallback)] // This can be removed after diesel-1.4
-#![feature(crate_in_paths)]
 
 extern crate activitypub;
 extern crate ammonia;
+extern crate askama_escape;
 extern crate bcrypt;
 extern crate canapi;
 extern crate chrono;