Rename bin/render 'grid2' command to 'grid'

- Also remove the old 'grid' command
bin/render: Handle cells in region w/o crimes

- In grid_for_region(), ensure that all grid cells within the region
  have a value of at least zero. This allows differentiation between
  ares for which we have no data (no regions cover them) and areas for
  which we have data but the data indicates that there have been no
  crimes there.
Update map description prose in region template
Move region pages to /r/ from /d/