Sort/group participations by section in disposition view

[jeriox]

For large shifts with a complicated structure, it can be hard to do disposition. This is due to the fact that participations are displayed as one long list of participations, sorted by creation date. It would be helpful to at least have them sorted by team/block. Another option would be to render the disposition view similar to the event detail view, enabling drag and drop into the corresponding structure element. (see also #1354)