Convert collaborators into teams

[jace]

Collaborators were introduced to Hasjob (technically named post admins) to allow more than one person to respond to a candidate. However, since collaborators need to be specified on each job, they present additional concerns:

1. Collaborators must be specified all over again on each post.
2. Posts continue to be owned by the user who posted them, who has no option to relinquish control. As a result of this, HR managers (a) make accounts in the company's name and (b) abandon these accounts when they move on.
3. A new HR manager cannot gain access to earlier posts.

Existing collaborators should instead be converted into Lastuser teams, and in future employers should be asked to (a) pick a collaborating team or (b) make one (using the existing widget).

Since teams must exist within organizations and posts don't have organizations (only domains), this issue depends on a resolution of either #141 or hasgeek/lastuser#151.