Consolidate district port/aerodrome unit checks and add additional resource & obsolence logic

[instafluff0]

A user pointed out that ports were enabling obsolete naval units to be built, which I realized also likely was an issue with air units. After reviewing the code, I noticed we also weren't checking for required resources in the given city.

I consolidated the logic a bit and added the resource and upgrade checks, including the special logic for units triggering golden ages from vanilla. Last, I noticed the code that read as:

// Superficially allow the AI to choose the unit for scoring and production.
// If a disallowed air/naval unit is chosen in ai_choose_production, we'll swap it out for a feasible fallback later
// after prioritizing the aerodrome/port to be built
if (! is_human && (
    (type->Unit_Class == UTC_Air || city_can_build_district (this, AERODROME_DISTRICT_ID)) ||
    (type->Unit_Class == UTC_Sea || city_can_build_district (this, PORT_DISTRICT_ID)))
)

... should read as (type->Unit_Class == UTC_Air && city_can_build_district (this, AERODROME_DISTRICT_ID)), not ||.

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Edit: it's been pointed out to me that naval and air units can't upgrade currently with ports and aerodromes and the *not_cities flags on. I'll work on that next, please hold off on merging for now.