[ticket/11352] Redirect the user back to view*.php after disapproving

The user should be redirected back to:
- the topic, if he came from viewtopic and the topic still exists
- the forum, if he came from viewtopic, but the topic does not exist anymore

PHPBB3-11352
[ticket/11352] Add functional tests for disapproving a post/topic

PHPBB3-11352
Merge remote-tracking branch 'github-nickvergessen/ticket/11352' into develop-ascraeus

* github-nickvergessen/ticket/11352:
  [ticket/11352] Add functional tests for disapproving a post/topic
  [ticket/11352] Redirect the user back to view*.php after disapproving