Add sites/within/count that returns the number of sites within a given bbox

[mraross]

This resource allows an application to know if a request for all sites within a given bbox will exceed the maximum results allowed.

parms include excludeUnits which excludes subsites and onlyCivic which includes only civic addresses

needed by GeoBC to inform their GIS analysts when there are more results they can't see.

[cmhodgson]

We have no shortcut to the child-count in our current index system, so requesting large bboxes can potentially be slow. We can reliably return whether the count is over the size of the maxResults in the same time as it would take to gather the maxResults, this is a minimal option to save time.