Log "Referer" header to help track tool usage

[melange396]

Extract and record the "Referer [sic]" HTTP request header (or, in its absence, the "Origin" header) to help us better and more easily measure and analyze the usage of our web application tools (like the COVIDcast Dashboard, for instance).

This can be accomplished in the server code here and here.

To get more granularity and thus utility out of this, we may also need to change the Referrer-Policy on our webapps (i have not investigated enough to know if this is actually necessary, but i strongly suspect that it is).

[melange396]

Lets keep this open for now, until we determine if we have to (and how to) deal with the "Referrer-Policy" stuff