Implement "No thanks" and conditional display

I went with a `rides_free` column on the participants table, with
default of NULL, that is set to TRUE when a user tips us and FALSE when
they explicitly opt out by clicking "No thank." This field isn't used in
the calculation of the percentage of supporters among active users; we
still use the actual transfers from last payday for that. This field is
just used in deciding to display the message to the user.