Makes it so that models that have maximum length validations will add
maxlength attributes to form fields. This includes flag reasons, appeal
reasons, and forum topic titles.
Partially fixes#4519 (Add "n/m characters remaining" character counter to the appeal reason).
https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/maxlength
* 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.
Regens simple form config using `rails generate simple_form:install`.
Seems to fix#2913 (erroneous minlength validations being enforced as of
simple_form 3.4.0).
