Understand or remove custom_web_context parameter of create_web_extensions_webview()

Currently create_web_extensions_webview() has a custom_web_context parameter. This allows some WebExtensions to run in their own special WebKitWebContext totally isolated from the rest of Epiphany. Such extensions get their own NetworkProcess, for example.

I'm not convinced this makes sense, and do not understand why some extensions use this but not others. This needs to be either reconsidered or, at least, better documented.