[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