Allow to use a custom gpodder server

[bohwaz]

Before posting

Please follow the steps below and check the boxes with [x] once you did the step.

• [X] I checked the issue tracker for similar issues
• [X] I checked the changelog if the issue is already resolved
• [X] I tried the latest Clementine build from here

System information

Please provide information about your system and the version of Clementine used.

• Operating System: Debian
• Clementine version: 1.4.0-rc1

Expected behaviour / actual behaviour

Currently using the GPodder sync service works fine but only connects to gpodder.net, but you cannot use a custom server.

It would be nice to be able to use our own custom server that is supporting the Gpodder API.