Support activation codes

To improve participant engagement, it would be nice to be able to provide users the ability to create an account as early as possible. As an example, a participant signs up on fundrazr. After doing so, they should receive an email noting that we've received their donation, and that they should expect an update on when their kit ships. It would be pleasant to provide them at this earliest point with a means to login.

The advantages are that we engage people early, and it creates a chance for them to take the surveys first and have a brief delay before dealing with the kit itself (and the FFQ). It helps to break up some of the process fatigue.

I think we can do this without too much pain. On the account creation page, we would need to modify the gate keeping to allow for testing for a valid activation code OR a valid kit ID (which has samples). We would not need to modify how samples are claimed.

Start at the account creation page and trace how the current kit-ID gate determines whether a participant may create an account. Verify the activation-code path and ensure account creation succeeds with either a valid activation code or a kit ID with samples, without changing sample-claiming behavior.

Written by the indexing model from the issue text.