Centralize logic for user/curator display name.

This logic is tricky due to GitHub's many (and optional) name fields.
I'm consolidating the scattered logic for this into a single support
method in the 'opentreewebapputil' module, which can be called from all
web2py apps.
Show preferred display name in curation header.
Improve display of feedback authors (names & URLs)

This takes advantage of our custom 'metadata' block and tries to
retrieve proper names, including for guest users.
Merge pull request #302 from OpenTreeOfLife/improved-display-name

Improved display names for users