Validation of cell.id duplicates

Extracted from #510193 (closed)

Do we need validation for the value of cell.id to make sure it does not clash with other cells ? How ?

A comment by @tkuah

The unknown is what happens when the Rails process does not like the cell id (either not a number, or overlaps with another cell). Making the rails process (web node, sidekiq node) not healthy seems like the only option. Raising an exception from config/initializer is another option.