add warning on name change

[chadwhitacre]

Rather than wait for Gittip to provide OAuth (#455), we're planning to use Gittip usernames as an integration point for, e.g., RubyGems.org (see rubygems/rubygems.org#500; also relevant to other partnerships done in this manner). This raises an issue when a Gittip user changes their username. At the least, it breaks the connection (someone following the link from RubyGems.org to Gittip would get a 404). If someone else grabbed the old username, then that person, whether maliciously or not, could siphon off tips intended for someone else.

We discussed various ways of addressing this, but they all sent us down the path of providing OAuth. That's a bigger project, so in order to get integration going sooner rather than later, we settled on throwing up a warning on Gittip name change: "Change your username elsewhere!" GitHub does something similar for repo name changes, e.g.

[chadwhitacre]

cc: @tehgeekmeister

[chadwhitacre]

This is done.