Refactor exporters to replace Laravel Excel with Simple Excel Writer

The exporters use a lot of memory currently – in fact, the all charities exporter (which isn’t used – we link to charitybase instead) 500s:
https://campaign-lab-constituency-explorer.c6digital.dev/exports/constituency-charities

I suspect this is also the reason for #26.

These exporters can be rewritten to use Simple Excel Writer, which is much more memory efficient.