Fix: Use page title for archive pages when WP page exists at same path

[rollybueno]

Summary

Fixes archive page titles to use WordPress page titles when a page exists at the same path, instead of always showing bbPress archive titles.

Changes

• Theme compatibility: Fixed topic archive to use $new_title instead of always calling bbp_get_topic_archive_title()
• Archive templates: Changed from bbp_forum_archive_title()/bbp_topic_archive_title() to the_title() to respect theme compatibility layer

bbPress Trac #3604