BO33 - Add the capability to change Sponsors' order

[gregoriopalama]

Modify the buttons added in #193 to change the order of a Sponsor.
When a sponsor is the first in the list, then the "Put up" button must be disabled.
When a sponsor is the last in the list, then the "Put down" button must be disabled.

Make sure that the list in the page is ordered by the position field for each sponsor.

When a sponsor's position is changed by clicking on a button, we will have 2 sponsors that need the position to change. The call to the Sponsorship API to update the position must be done for the sponsor with the lowest position value: this is because the endpoint will update the position for the other sponsor too.

Depends on #187 and #193.