Make the edit action default to z17

If an attempt is made to invoke an editor without specifying a zoom
level (as with the GPX edit links) then default to z17 instead of z14
to avoid trying to load too much data. Fixes #3728.

app/controllers/site_controller.rb

@@ -64,7 +64,7 @@ class SiteController < ApplicationController
         #Currently this results in potlatch starting up at 0,0 (Atlantic ocean).
       end
 
-      @zoom = '14' if @zoom.nil?
+      @zoom = '17' if @zoom.nil?
     end
   end
 end