More characters in nicknames

Here is the correct way to do nicknames:

1-64 alphanum chars in any UTF-8 script + ".", "_", "-" (see Brenda's great tag regexp for a good example)
canonicalize nicknames by lowercasing and removing punctuation (again, like tags)
save "display" and "canonical" in user table ("BUGabundo" and "bugabundo", "evan_d" and "evand")
look up using canonical
display using display version
Don't allow registration when the canonical already exists
Allow reference in text with anything that canonicalizes to canonical ("@Evan", "@eVaN" and "@e_v_a_n" all work)
Don't (DON'T!) change the text of @replies!

Note that this change requires a database addition (I think we just add a "display_nickname" column).