Update rocket_i18n and add gettext_macros (#431)

Internationalization now uses proc-macros that generate the .pot file
automatically.

templates/errors/base.rs.html

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

templates/errors/csrf.rs.html

@@ -3,10 +3,11 @@
 
 @(ctx: BaseContext)
 
-@:base(ctx, "", {
-    <h1>@i18n!(ctx.1, "Invalid CSRF token.")</h1>
-    <p>@i18n!(ctx.1, r#"Something is wrong with your CSRF token.
-Make sure cookies are enabled in you browser, and try reloading this page.
-If you continue to see this error message, please report it."#)
-</p>
+@:base(ctx, i18n!(ctx.1, "Invalid CSRF token"), {
+    <h1>@i18n!(ctx.1, "Invalid CSRF token")</h1>
+    <p>
+        @i18n!(ctx.1,
+        "Something is wrong with your CSRF token. Make sure cookies are enabled in you browser, and try reloading this page. If you continue to see this error message, please report it."
+        )
+    </p>
 })

templates/errors/not_authorized.rs.html

@@ -1,7 +1,7 @@
 @use templates::errors::base;
 @use template_utils::*;
 
-@(ctx: BaseContext, error_message: &str)
+@(ctx: BaseContext, error_message: String)
 
 @:base(ctx, error_message, {
     <h1>@i18n!(ctx.1, "You are not authorized.")</h1>

templates/errors/not_found.rs.html

@@ -3,7 +3,7 @@
 
 @(ctx: BaseContext)
 
-@:base(ctx, "Page not found", {
+@:base(ctx, i18n!(ctx.1, "Page not found"), {
   <h1>@i18n!(ctx.1, "We couldn't find this page.")</h1>
   <p>@i18n!(ctx.1, "The link that led you here may be broken.")</p>
 })

templates/errors/server_error.rs.html

@@ -3,7 +3,7 @@
 
 @(ctx: BaseContext)
 
-@:base(ctx, "Internal server error", {
+@:base(ctx, i18n!(ctx.1, "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>
 })

templates/errors/unprocessable_entity.rs.html

@@ -3,7 +3,7 @@
 
 @(ctx: BaseContext)
 
-@:base(ctx, "Unprocessable entity", {
+@:base(ctx, "Unprocessable entity".to_string(), {
   <h1>@i18n!(ctx.1, "The content you sent can't be processed.")</h1>
   <p>@i18n!(ctx.1, "Maybe it was too long.")</p>
 })