It is not uncommon for forums to be intergrated with sites where users
don't have a unique "handle" - they might just have their first name,
or a full name, which is not guaranteed to be unique.

This commit introduces the concept of "display names" for users. By
default display names are the same as usernames, but extensions may
override this and set them to something different. The important thing
is that all code should use `display_name` whenever intending to output
a human-readable name - `username` is reserved for cases where you want
to output a unique identifier (which may or may not be human-friendly).

The new "GetDisplayName" API is probably sub-optimal, but I didn't worry
too much because we can come up with something better in `next-back`.

ref #557

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