Remove items via ajax

If the user removes an item on the unread, bookmark or history page, it is now done via an ajax call and the page-counter, nav-counter and page-title gets updated.

While implementing the feature i've fixed additionally the following bugs/inconsistencies:
- unify page-counter on all pages which allows javascripts to increase/decrease the counter on any page
- decrease the page/title counter on the history page if an item is marked as unread
- fix lowercase Miniflux pagetitle in javascript function hideItem()