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

build.rs

@@ -0,0 +1,15 @@
+extern crate ructe;
+extern crate rocket_i18n;
+use ructe::*;
+use std::{env, path::PathBuf};
+
+fn main() {
+    let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
+    let in_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap())
+        .join("templates");
+    compile_templates(&in_dir, &out_dir).expect("compile templates");
+
+    println!("cargo:rerun-if-changed=po");
+    rocket_i18n::update_po("plume", &["de", "en", "fr", "gl", "it", "ja", "nb", "pl", "ru"]);
+    rocket_i18n::compile_po("plume", &["de", "en", "fr", "gl", "it", "ja", "nb", "pl", "ru"]);
+}