Move Snowflake proxy page somewhere devs can write

The Snowflake badge code is hosted at serene's site, https://keroserene.net/snowflake/embed.html. serene is the only one who can modify files there. It would be more convenient to move the code to another web server to which all the developers can push files.

Like in the past we moved the flash proxy files from https://crypto.stanford.edu/flashproxy/ to https://flashproxy.bamsoftware.com/flashproxy/. We could even use the same server for Snowflake (using a Snowflake-related domain name).