User account management from CLI

[aliaksei135]

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What does this PR do and why is it necessary?

Adds User account management from the command line through the user command group.
Commands added are add_user, remove_user and change_password.

How was it tested? How can it be tested by the reviewer?

Manual testing of commands, nosetests all pass.

Any background context you want to provide?

What are the relevant tickets if any?

Solves #1239.

Screenshots (if appropriate)

Further notes

Server needs a reboot after any user commands to load new users.yaml.
Needs documentation and updating of "Forgot password" link on login page once documented.

[foosel]

That is a great idea! I have two minor issues, apart from those this is ready to merge! 👍

[foosel]

We forgot something

[foosel]

👍