Enhancement/reorder categories

[henriquecbuss]

What issue does this PR close

Closes #780

Changes Proposed ( a list of new changes introduced by this PR)

• Fix category reordering by using the position field
• Recalculate the trees on the frontend for optimistic updates

How to test ( a list of instructions on how to test this PR)

• Go to the categories settings page
• Try to reorder them by dragging and dropping, and by using the "move up" and "move down" buttons

Still Pending

• While all the functionality is already done, the UI still needs some work
• The backend is having some issues with reordering root categories

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