I had a bad unique index on the `cells` table. This commit does not fix
it but has two backfill scripts that will allow me to fixup the data
before creating the new unique index.
`is_base_game` identifies cells that are from the `Skyrim.esm` shipped
with the game (and not from some mod that happens to also have a
`Skyrim.esm` with a 0,0 cell with a different form id, see:
https://github.com/thallada/modmapper-web/issues/8).
`dump_cell_data` now only dumps cells from `is_base_game` cells.
`deduplicate_interior_cells` removes duplicate cell and plugin_cell rows
caused by the buggy index.
I will need to upgrade the database and create a new index in a later
commit.
