Implement OnDesk purchase process and extend checkin system

[MarkusH]

• [x] On desk purchase process
• [x] Purchase detail view
• [x] Change purchase state
• [x] Download invoice
• [x] Update ticket
• [x] Download badge
• [x] Link ticket actions (cancel, edit) from purchase detail view

[MarkusH]

@DasIch, @zerok Can you please review this PR

[led02]

Looks fine to me (except comment above) and works in my testing environment.

[MarkusH]

Thanks @led02 . I found a bug with the real pdf generator which I, unfortunately, don't know how to test (32a0202)