* Fix it so that all edit forms show an error banner if the form
has validation errors. Previously forms had to manually call
`error_messages_for`, which not all forms did.
* Fix it so that the full validation error message is shown next to each
input attribute that had errors. Also update the styling of these
error messages to look better.
* Refactors DText form fields to use a custom SimpleForm input instead
of manually generated html. This fixes it so that DText fields use the
same markup as normal SimpleForm fields, which lets us apply browser
maxlength validations to DText input fields.
* Fixes autocomplete for @-mentions only working in comments and forum posts.
Now @-mention autocomplete works in all DText fields, including dmails.
Known bug: it applies in artist commentary fields when it shouldn't.
