(bug 43661) Only construct trail for simple links

* [[foo|foo bar]] no longer roundtrips to [[foo]] bar
* [[foo]]s still RTs to [[foo]]s
* [[foo]]s modified to be [[foo]]ls will still RT to [[foo]]ls
* [[foo]]s modified to be <a href="./Foo">fools and stuff</a> will now RT to
  [[foo|fools and stuff]]

Change-Id: I385afabab93126d6a6ddf8703619d72ed106b66d
Merge "(bug 43661) Only construct trail for simple links"