[ticket/12153] Assign PAGE_NUMBER in pagination.generate_template_pagination()

PHPBB3-12153
[ticket/12153] Remove vars already assigned in generate_template_pagination().

PHPBB3-12153
[ticket/12153] Remove redundant calls to pagination.on_page().

PHPBB3-12153
[ticket/12153] Adjust pagination tests.

PHPBB3-12153
[ticket/12153] Remove duplicate assignment of $u_previous_page & $u_next_page.

PHPBB3-12153
Merge pull request #2002 from prototech/ticket/12153

[ticket/12153] Assign PAGE_NUMBER in pagination.generate_template_pagination()