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/server_error.rs.html

@@ -0,0 +1,9 @@
+@use templates::errors::base;
+@use template_utils::*;
+
+@(ctx: BaseContext)
+
+@:base(ctx, "Internal server error", {
+  <h1>@i18n!(ctx.1, "Something broke on our side.")</h1>
+  <p>@i18n!(ctx.1, "Sorry about that. If you think this is a bug, please report it.")</p>
+})