Add more usernames to blacklist

I noticed that root and postmaster were missing, one thing led to
another, and I added a bunch of names from this list:

    https://programmers.stackexchange.com/questions/115425/is-there-a-list-of-common-usernames-to-reserve-in-a-new-system
Unblacklist a few names

As @tilgovi points out, "nick" probably shouldn't be blacklisted. (An
honest mistake, guvnor! I promise!)

Also, java{,script}, ruby, php.
Merge pull request #1576 from hypothesis/username-blacklist

Add more usernames to blacklist