LocalSiteAwareModelFormMixin uses the LocalSite ID passed when

posting form data and validates all other relation fields against that.

An assumption is made that we can perform a query for a LocalSite

given that ID, and on some databases that works fine, but on others it

will raise an exception if the ID isn't numeric.
We now check for this ahead of time and ignore the value if provided,

avoiding a confusing crash.

Unit tests pass.
Verified the crash occurred without this fix but was fixed with it.