implement hard "no tips" option

[chadwhitacre]

I implemented a soft "no tips" option as part of #114. But for reasons first articulated on #80, we need to implement a hard option. The difference between soft and hard here is that for the soft option we still move money, the person just states an intention to regift, whereas for the hard option we wouldn't actually move any money to the person.

The two reasons we need this are:

• We want to avoid burdening people who want to give and not receive on Gittip with the busy work of making sure their gifts out exceed their gifts in.
• We need to let people pick arbitrary participant_ids, and we want to support the case where someone wants to claim their username in order to prevent tips to it. Actually, in that case we should probably require that the name be active on one of the social networks. Hmm ...

[ncoghlan]

Wouldn't regifting percentages of incoming tips also cover the first reason? (i.e. preventing accumulation of a gittip balance)

[chadwhitacre]

This has been implemented for a while.