Add tags for certain categories of requests

[cecilia-donnelly]

Add a functionality whereby a "tag" can be added to a request based on a url parameter. So, a new version of the site at saf.getasmokealarm.org would have the saf tag appended to any requests that came through it. This will allow administrators to count the number of certain kinds of requests that are received through the system, in order to manage certain goals.

Presumably, counts of tagged requests should then be available in some kind of report. We don't yet know how exactly that information should be presented.