feat(themes): remove themes

See #143 for more info.

Remove `vk_skinman.js` completely, and all the things regarding
custom site themes over other scripts (e.g. settings).

This closes #27, closes #61, closes #143.
feat(left menu): call UpdateCounters() on hover

- call UpdateCounters() on left menu hover
- remove context menu item "Refresh menu"

This closes #98.