Filter tag site-wide searches to just tag results

[mrueschman]

Along with #440, when on a variable page, clicking a label should do a site-wide search that is filtered to tags (e.g. "from=tags" parameter)

Click health_interview_shhs, destination URL might look like: https://sleepdata.org/search?from=tags&search=health_interview_shhs (right now "from" can be blog, forum, variables, datasets)

[remomueller]

I'm not quite sure what the difference would be for tags. This button currently goes to the "All Results", did you want it to go to the "Variables" results instead? like https://sleepdata.org/search?from=variables&search=health_interview_shhs