Improve compound node labels in tree viewer.

Show the proposed '[Taxon A + Taxon B]' format for (display) leaf nodes,
page titles, and in the property inspector. URLs will use slightly
modified version with web2py-safe characters. Do *not* show label for
(display) internal nodes in the tree view, to avoid clutter and
confusion.

N.B. This does not include proper leaf counts, since we need this from
treemachine. Same for "upward" nodes from the target, since we don't
have data in the view to create these labels yet.
Merge pull request #243 from OpenTreeOfLife/compound-node-labels

Improve compound node labels in tree viewer.