Disable the browser's native spellchecking ability on all form inputs,
except for DText inputs. We do this by setting `spellcheck="false"` on
the <body> tag, and `spellcheck="true"` on DText <input> tags.
This fixes browsers displaying a red wavy underline beneath tags in the
tag search box, among other places. We disable spellchecking globally
because most form inputs, except for DText inputs, aren't meant for
natural English language.
* 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.