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

templates/errors/base.rs.html

@@ -0,0 +1,9 @@
+@use templates::base as base_template;
+@use template_utils::*;
+
+@(ctx: BaseContext, error_message: &str, error: Content)
+
+@:base_template(ctx, error_message, {}, {}, {
+  @:error()
+  <p>@error_message</p>
+})