Make slots and rooms sortable via drag and drop

[daniel-fink-de]

In this PR, I have implemented a potential solution for making slots and rooms sortable based on #13.

The changes can be summarized as follows:

• Add an int OrderNumber property for the Slot and Room entities, respectively.
• Create additional event handlers with interact.js to accommodate the drag-and-drop functionality.
• Update the target and source Slot and Room, respectively, via switching the corresponding OrderNumber property.

Please note:

• When a new Slot or Room is created in the backend, the value of OrderNumber will be set to the maximum number of all existing slots and rooms plus one, respectively.

Potential improvements:

• The OrderNumber may be shown and edited in the room and slot modal, respectively.