Allow using value from preferences in args

In the args of a template, we should allow using an arbitrary preference.

For example, for the e-mail field, we probably want to have a preference giving the default mail domain. This is not even specific to the CSV import, such a preference would be useful in AlekSIS globally. But then, we do not want to hard-code the domain in the template, but use the preference instead.

The idea is to allow some kind of formatter in strings, like it is done in DJango's URL route parsing, or in format strings, or… Example: {preference:foo}