Remove dependency on Kernel Gateway

[kevin-bates]

In order to move forward with some roadmap items and open issues that require alterations to kernel and kernelspec handlers, it makes sense for Enterprise Gateway to remove its dependency on Kernel Gateway. At quick glance, this would essentially amount to copying the websocket, base, mixin handlers and some config traits from KG to EG - in addition to some doc updates.

[kevin-bates]

The following issues may have been prevented from moving forward due to an inability to affect request handlers: #555, #562, #594, and #731. With these changes (in #734), we should be able to have flexibility to address these issues.