astakos: Add function to send plain emails

With the `send_plain` function, the admin can send a fully customizable
mail to an AstakosUser. The admin can either choose to use the provided
body template (`plain_email.txt`) and subject, or write his/her own.
astakos: Add check function for project actions

Add a `validate_project_action` function that takes a project and an
action as arguments and checks if the action can be applied on the
project.

This function has been created from the existing checks that each
project action has. Its main purpose is to export this project logic and
provide a common endpoint for any module that wants to use it.
astakos: Add logic for users

Add an astakos logic module that serves as a common endpoint for actions on
users. This module acts as a wrapper above `activation_backends` and is
currently used by `snf-manage`.
astakos: Add tests for user logic