Redesign new article / edit article page

The new article / edit article page needs to be redesigned to look similar to the article display page, only with editable fields. Category editing needs to be revamped as the current UI is horrible.

Start by locating the new article and edit article page entry points, then compare them with the article display page. Review the current category editing UI and define the redesign around editable fields, a display-page-like layout, and a clearer category editor; done means both article forms match the intended presentation and category editing is improved.

Written by the indexing model from the issue text.