[ticket/12613] Improved pagination

PHPBB3-12613
[ticket/12613] Fix dropdown-up positioning

PHPBB3-12613
[ticket/12613] Fix tests

PHPBB3-12613
[ticket/12613] Removed all previous_page / next_page text links

PHPBB3-12613
[ticket/12613] Removed jump-to placeholder and added .focus() check

PHPBB3-12613
[ticket/12613] Fix functional test and post sorting bug

PHPBB3-12613
[ticket/12613] Removed form in order to prevent nested forms

PHPBB3-12613