Ensure the correct FK field name is used in link table rules

[pimterry]

This code assumes that the FK from a link table to another table has the same name as the target table. That's sometimes true, but not always - nowadays we include the verb in there. This breaks some parts of the billing model.

This code now searches the fields for a FK that connects to the correct table, which fixes this issue.

There's a further extension to this where we find the FK for the correct verb, since there could be multiple. I haven't looked at that because a) it doesn't block me and b) I can't see an easy way to connect the fields to the verbs without writing special logic to parse verb & field names, which seems pretty nasty. Any ideas?

Connects to #18
Change-Type: patch

[ghost]

@pimterry, status checks have failed for this PR. Please make appropriate changes and recommit.

[pimterry]

Forgot to update the tests correspondingly - now fixed (though I question the value of a test suite that needs bug fixes that are near-identical to the real code...)

[Page-]

It took a bit of time to figure out how to resolve the relationship correctly in the lf-to-abstract-sql context (it's only been used in other contexts previously), but it turns out it's actually really simple, here's the code I used:

	LinkTable :actualFactType =
		RoleBindings(actualFactType):binds
		LinkTableAlias(binds, actualFactType):tableAlias
		GetTable(actualFactType):linkTable
		{['SelectQuery', ['Select', []], ['From', [linkTable.name, tableAlias]]]}:query
		{	_.each(binds, function(bind, i) {
				var baseIdentifierName = actualFactType[i*2][1],
					targetTable = $elf.GetTable(baseIdentifierName);
				if(!targetTable.primitive) {
					var relationshipMapping = $elf.relationships[linkTable.name][baseIdentifierName].$;
					$elf.AddWhereClause(query,
						['Equals',
							['ReferencedField', tableAlias, relationshipMapping[0]],
							['ReferencedField', bind.binding[1], relationshipMapping[1][1]]
						]
					);
				}
			})
		}
		-> ['Exists', query],

Basically a relationship mapping is [ $fromField, [ $toTable, $toField ] ] (or just [ $fromField ] if it's a primtiive attribute - which it won't be here). If you could check that out and see if it still works for you that'd be great, it passes the tests at least