Allow buyer to assign ticket to other user

[MarkusH]

We will generate the conference badges from the sold tickets and add additional information from the assigned user profile. In case the buyer is the attendee himself/herself this is not a problem. But in situations where someone bought additional tickets for eg. colleagues this is not feasible anymore. Hence allow the buyer to assign a ticket to a user (profile). But don't allow users to revoke the assignment. This should only be able through the helpdesk.

Related to the above, allow a buyer and assignee to change certain properties of a ticket (eg the t-shirt size, but not the name).

[zerok]

@Markush2010 I'd prefer to put the whole "additional ticket fields" stuff into a different ticket to keep tickets as small as possible. Will create one later to do just that, if you don't mind.

[zerok]

Second part externalized into #106