simple form: refactor DText form fields to use SimpleForm.

* 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.

app/javascript/src/styles/common/simple_form.scss

@@ -39,7 +39,7 @@ form.simple_form {
       padding-left: 1em;
     }
 
-    &.text {
+    &.text, &.dtext {
       .hint {
         padding-left: 0;
         display: block;