Should not be able to start ProxyScan when nothing is in scope

[mckinney-subgraph]

Currently it is possible to start a proxy scan when nothing has been added to the scope. But because nothing has been added to the scope, the proxy scan will not go anything. This may be a source of confusion for the user, who may assume that it should be scanning all of the pages they are visiting.

There should be an indicator in the GUI to show the user that they must add some URLs to the scope for the proxy scan to work. The simplest way to do this would probably be to just grey out the "Start Proxy Scan" button until something has been added to the scope.

Another approach may be to let the user start the proxy scan but prompt them to add the page they are browsing to via the proxy to the scope. The prompt could occur if the user browses to a page for the first time and there is nothing in scope, or it could occur each time the user browses to something that is not in scope.