feat (lookups): Support hierarchical lookups

Some lookups fall into natural hierarchies: esp Islands => Districts. In particular, a school belongs to a district by virtue of the Island to which it is assigned.

Therefore, in the School Item display, we need to decode the District code and district name from the island code - and do that dynamcially if iCode changes.

So, we'll add a field D to the islands lookup to get the district ID, this can be passed to the Districts lookup to get the district name.

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