Better big form handling (#430)

* Allow customizing max form size from env vars

* Add error page for unprocessable entities

And change default http port to 7878

* Improve char counter: under the editor, more discrete, and give it a default value

templates/errors/unprocessable_entity.rs.html

@@ -0,0 +1,10 @@
+@use templates::errors::base;
+@use template_utils::*;
+
+@(ctx: BaseContext)
+
+@:base(ctx, "Unprocessable entity", {
+  <h1>@i18n!(ctx.1, "The content you sent can't be processed.")</h1>
+  <p>@i18n!(ctx.1, "Maybe it was too long.")</p>
+})
+