Put an opt-in button on the main snowflake page

When you go to https://snowflake.torproject.org/snowflake.html for the first time, you get the message "Not opted-in." You have to go to https://snowflake.torproject.org/options.html, click "Yes", then go back to snowflake.html before you start being a proxy. (options.html doesn't have the proxy code.)

The discoverability at https://snowflake.torproject.org/index.html is worse: it doesn't say anything about opting in: you have to intuit that you should click on the badge to get to options.html, then go back again before you start being a proxy.

I propose that both these pages should have a Yes/No button right there on the page. Clicking Yes should do both:

1. set the opt-in cookie
2. start up the proxy (without needing to refresh the page) Additionally, options.html should have a copy of the proxy code, so after you click Yes, it starts working immediately, without having to go to another page.