Add scanpost

Admins should be able to check things in and out on the day itself. The following workflows can be distinguished:

#### Checking out
* A partner wants to check out an item
* He shows his barcode
* The admin scans the barcode and then the items barcode
* The system checks if the partner has an item reserved
* If yes:
  * The system checks if the partner has still items which are not picked up
    * If yes: 
      * Increment the picked up counter with 1
      * Display a success message
    * If not: display a warning that the partner has reached his limit for this item
* If not: display a warning that the partner didn't make a rseservation
* If a warning is displayed and the admin agrees:
  * If there is a reservation:
    * Increment the count value 
    * Increment the picked up value
  * If not: create a new reservation
  * Show a success message

#### Checking in
* A partner wants to check in an item
* He shows his barcode
* The admin scans the barcode and then the items barcode
* The system increments the brought back value
* TODO: What happens when the partner brings back an item which he didn't reserve?

TODO:
- [x] Add barcodes to partners
- [x] Add barcodes to items
- [x] Add `picked_up_count` and `brought_back_count` to registrations
- [x] Create the scanpage
- [x] Make sure partners can download their barcode again
- [x] Add printable reports for items
- [x] Add new fields to papertrail and display them in the overview
- [x] Code procedures defined above
- [x] Tests