Add support for creating, deleting, listing, showing users

[fishnix]

This was a bit more work than I anticipated... so far this supports:

• listing users
• creating users
• showing details about a user
• deleting users

TODO:

• reseting access keys
• filling in details about groups and access keys when showing details about a user
• fill in some tests

[damnski]

Looks really good. A huge commit. Just a couple comments.

[tenyo]

Good stuff! A few comments/questions 👍

[tenyo]

Excellent! 👍