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-common/src/lib.rs

@@ -1,4 +1,4 @@
-#![feature(custom_attribute, iterator_flatten)]
+#![feature(custom_attribute)]
 
 extern crate activitypub;
 #[macro_use]
@@ -10,7 +10,6 @@ extern crate chrono;
 extern crate failure;
 #[macro_use]
 extern crate failure_derive;
-extern crate gettextrs;
 extern crate hex;
 extern crate heck;
 extern crate hyper;