Increase threshold for minimum polygon size

We filter out very small polygons from the original data to remove
glitches. These glitches are caused by trying to subtract the water from
a polygon that includes some land and some water, but using two
different definitions or resolutions of mean high water line.

If we don’t do this then we end up with a bunch of very small polygons
which lie far outside the understood area of a place, causing large
overspill.

We need to increase the threshold for this process because we’re still
seeing this problem around Bristol and Norwich.

This does mean we lose a few very small polygons in places like Shetland
and the Scilly Isles, but not in such a way that we would avoid
broadcasting to them (because they’d still be caught by the
simplification and overspill).

app/broadcast_areas/polygons.py

@@ -41,7 +41,7 @@ class Polygons():
     # The threshold for removing very small areas from the map. These
     # areas are likely glitches in  the data where the shoreline hasn’t
     # been subtracted from the land properly
-    minimum_area_size_square_metres = 50 ** 2
+    minimum_area_size_square_metres = 14_000
 
     def __init__(self, polygons):
         if not polygons: