[ticket/8610] Merging topics does not handle bookmarks correctly

https://tracker.phpbb.com/browse/PHPBB3-8610

PHPBB3-8610
[ticket/8610] Fix bookmarks when merging all the posts of a topic

PHPBB3-8610
[ticket/8610] Throw an error when the dest topic isn't set

PHPBB3-8610
[ticket/8610] Add functional test when two topics are merged

PHPBB3-8610
[ticket/8610] Use css selector instead of xPath

PHPBB3-8610
Merge pull request #2434 from Nicofuma/ticket/8610

[ticket/8610] Merging topics does not handle bookmarks correctly

* Nicofuma/ticket/8610:
  [ticket/8610] Use css selector instead of xPath
  [ticket/8610] Add functional test when two topics are merged
  [ticket/8610] Throw an error when the dest topic isn't set
  [ticket/8610] Fix bookmarks when merging all the posts of a topic
  [ticket/8610] Merging topics does not handle bookmarks correctly