Incorporate Santa Clara County boundary changes layer into OpenHistoricalMap

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The Santa Clara County Planning and Development Department maintains a GIS layer of boundary changes that includes annexations and “reorganizations” that affect the extent of unincorporated areas within the county. This dataset is in the public domain as a work of a California government agency. Unlike the San José layer in #28, this dataset doesn’t seem to be comprehensive, but for example, it does include boundaries like Palo Alto’s original boundary in 1894 and most (all?) of Milpitas’ annexations. Maybe we’ll be able to piece together boundaries for these cities analogous to the ones we imported for San José.