Proxy URL for server

[gdey]

#17
One of the urls listed in #17 is the proxy. We are pulling that out of #17 into this issue.

Changes to the config:
Webserver section should now have an additional config called proxy_path which is a string.

[webserver]
   proxy_path='http://localhost:9090/sheets/{{.SheetName}}/files/mgdid/${{.MGDID}}.pdf'

This is what will be filled into the pdf_url field for info.
It will disable the querying the filestores for urls.
This will enable the /sheet/${sheetname}/files/mgdid/${mdgid}.pdf end point on the server.
This end point will query the filestores for an io.Reader used to proxy the file.