Assign requests by waiting time or similar

[bjendres]

Currently, the list of open help requests (people still needing help) for the matching is generated without any particular order. Especially if the help demand massively outweighs the supply (offers), it might be sensible introduce some kind of order, e.g.

• oldest requests first (that would require a new custom field)
• oldest contacts first (simply order by contact_id), but this has some drawbacks
• ...