Need to encode URLs for schemes that use query parameters

[quinncomendant]

For a podcasts with a URL such as:

http://example.com/feed?id=123

This is fine when used with a simple app scheme, like this:

podcast://example.com/feed?id=123

However, if the app uses a scheme with query parameters such as overcast://x-callback-url/add?url= the URL doesn't work. In the following example, it does not invoke the Overcast subscribe callback, I think because of the double ?, so part of the query is ignored:

overcast://x-callback-url/add?url=http://example.com/feed?id=123

To fix this case, the podcast URL should be encoded at least as much as this:

overcast://x-callback-url/add?url=http://example.com/feed%3Fid=123

Or probably safer to encode the entire URL:

overcast://x-callback-url/add?url=http%3A%2F%2Fexample.com%2Ffeed%3Fid%3D123

This needs to be done in clients_panel.coffee to determine if the URL needs encoding or not (because if a simple scheme is used, e.g., podcast://, URL encoding shouldn't be done).