feature: Add a management command to add users from a file.

This adds the “add_user” command, which will add a number of users from a text file. The file is supposed to contain one user per line, where the format is

FIRST NAME:LAST NAME:EMAIL ADDRESS:LANGUAGE

and LANGUAGE is either D (German) or E (English).

Users will be added only if their e-mail address doesn't already exist in the database. With the --invite option, an invitation e-mail will be sent to each user's given e-mail address as the user is being processed (even if the user already exists in the database). The --template option allows the administrator to specify a template to be used for the invitation; if this is called template.txt then the system will check for template_de.txt or template_en.txt before it looks for template.txt.