Do not use the default WebKitWebContext

Create a global WebKitWebContext in EphyEmbedShell and use it everywhere
as the default web context. This is required to provide constructor
properties to the web context.
embed: Use a custom local storage directory for epiphany

Instead of using the default path. This ensures private profiles don't
leave data in the user home directory.

https://bugzilla.gnome.org/show_bug.cgi?id=738720