Import users from csv file

[fisx]

2015-06-29-mf-csv-ftp-prototype (WARNING: I just rebased this branch!) contains relevant code.

We may need this for our september deadline. Assume the following: Adhocracy backend is running in production and has an existing user base. We will re-deploy with thentos as a proxy, but the deployment needs to keep the existing user base, plus allow for create new ones locally, plus allow for other authentication methods.

There may be a better way to get user data from the adhocracy backend (rest requests?). If so, it's ok to change this task.

[fisx]

@robx just brought up an interesting point: we also need to inherit passwords from adhocracy. options:

1. make sure that we use the same scrypt settings so we can import and use the hashed passwords.
2. email password rest links to all imported users with a hand-crafted invitation email.

the first option would be much better in terms of usability, of course.