Email campaigns

[jace]

Similar to web campaigns, Hasjob needs an email campaign feature with the same targeting criteria, but with some differences:

1. A "campaign category" item needs to exist, meant for recording user preferences for that category. This is analogous to an email list that maintains subscriptions and unsubscriptions, but in this case only recording explicit user activity such as an unsubscribe.

2. The target list becomes a list of email addresses and phone numbers (where available), with the option of either the user's account email (which may be a personal account) or the jobpost email (which may be a shared email account and only for employers, with a possible conflict with #364 and #375).

3. Users who have unsubscribed from the category are always excluded from the targeting criteria.

Emails campaigns can be of two sorts (this differentiation is a pending feature in web campaigns).

Standing campaigns

Any user who meets the targeting criteria at any time in the future (while the campaign is valid) will receive the email. The campaign includes a periodicity test (once a day, etc) that looks for new candidates to include.

One off campaigns

The target list is prepared once and emailed just once. The email can be sent via Hasmail allowing for an unsubscribe link going back to Hasjob. (In built recognition of unsubscribe links and bounce reporting may be useful features in Hasmail.) The campaign may also be sent entirely within Hasjob by replicating some of Hasmail's message composer functionality, but a standardised API is desirable as it can also be used in Lastuser and Funnel.

This issue deprecates #46.