Template cleanup

A few updates to templates to simplify markup, remove legacy classes and make customisation easier:

 * move `table thead tr.row_tablehead` to `table caption`
 * remove explicit `row_odd`, `row_even` and `row_hover` classes (and hover.js script), now handled by css
 * use `for ... empty` in templates whenever possible
 * remove `row_columnheads` class from table headers
 * separate dashboard elements (summary, status, ticket lists) to allow easier customisation (e.g. adding columns to ticket lists)