Admin API - Delete API User

[brylie]

Create a REST endpoint in the Admin API for deleting API Users / API Keys.

User Story

As a developer extending API Umbrella Web
I need to delete users via the Admin REST API
so that I can ensure the user collection is consistent with my application

[brylie]

Possibly related to #134

[sandeepkottagundu]

DELETE API USER issue is open or closed

[brylie]

This issue is still open, but may have been resolved. @sandeepkottagundu, what difficulties are you having?

[aniketgambhire]

@GUI You can delete API User with the help of delete operation of mongodb. But using rest api you can't.

[brylie]

Ping @GUI