edit form: no fixed size for textarea

[ikiwiki] / doc / todo / edit_form:_no_fixed_size_for_textarea.mdwn

At the moment the text area in the edit form has a fixed size of 20 rows.

On longer pages its not very comfortable to edit pages with such a small box. The whole screen size should be used instead([example](http://img3.imagebanana.com/img/bl10u9mb/editingtodo_1241804460828.png)).