Feature request - Allow nested groups or global search across all wiki spaces

[volkswagner]

The limitation of a single group (parent/child) relationship is too limiting. To better organize the wiki, we need ability to create nested groups or when creating additional Wiki spaces, add the ability to search all wiki spaces (rather than the currently displayed wiki space).

[joshuarestivo]

Seconding this and also mentioning this other issue as it's related: #195

Navigation is currently too simplistic to be very useful.