clarifications for username resets

[cellio]

We previously updated the TOS in #53 but never deployed the change. In reviewing the draft announcement of the change we noticed a couple points that were still open to interpretations we did not intend.

Changes here:

• Clarify that if we change your username it will only be to reset it to the default; we do not assert the right to rename you to something you never agreed to. I also tried to make the wording here a little more direct.
• Add that if your account goes away, we're free of attribution obligations (since, you know, we can't name/link an account that isn't there any more).
• Where we said that we satisfy attribution by showing your name, added the link to the profile that (a) we provide anyway and (b) I believe CC attribution clauses require. (I don't feel strongly about this point so if others disagree we can undo this part. I put it here so we wouldn't look like we were trying to shirk a reasonable method of attribution.)