Form error state overhaul

This adds a new FormState structure here to hold the state of an errored
from, including retaining field value and better error codes.

It also adds error recovery for the registration form, and properly
loads the post_login_action context in case of errors.

crates/data-model/src/lib.rs

@@ -22,7 +22,6 @@
     clippy::trait_duplication_in_bounds
 )]
 
-pub mod errors;
 pub(crate) mod oauth2;
 pub(crate) mod tokens;
 pub(crate) mod traits;