Allow notification about job offers

[MarkusH]

We want sponsors to be able to send job offers to those users who explicitly except them (opt-in including later ability to deactivate the option again). But since we don't want to expose any user information to the sponsors, we need some kind of HTML offering the possibility. The check-box to receive / not receive offers should first be part of the registration form (deactivated) and second be accessible through the "change your profile" page

A limited set of users (maybe a Django permission) should eventually be able to post a text that is then send to all users having the respective flag turned on in their user interface.

I suggest to let this code live in pyconde.sponsorship