[Bug 928899] Add login tokens to answer emails.

This adds two things

1. A new authentication backend that reads auth tokens from query string parameters and logs users in based on that. The tokens are the same tokens that are used for password reset, plus a user name, base 64 encoded. It's a little ugly, but I think it works well and is safe.

2. It adds some of those tokens to the url that gets sent to a user when a contributor sends an answer.

I can't get real emails to work, so I haven't thoroughly tested the second part, but I did test the first part manually pretty well.

This might need some unit tests for part 1, but I haven't thought of how to test it yet.

r?