Adjust zoom ranges of subway, light rail & tram

[nvkelso]

In the transit layer, let's show subway earlier at zoom 8 (now 10), and light rail and tram to zoom zoom 9 (now 10).

• Related national rail zoom changes: Categorize regional & national public transit trains #471
• Related to funicular zoom changes: Add funicular & monorail to transit layer #588

In the mz_calculate_transit_level SQL function:

• Update zoom for subway to 8
• Update zoom for light_rail and tram to 9
• Do we need to update the GIST index in apply-planet_osm_line.sql?

UPDATE: subway was requested at zoom 7. On second thought I think 8 would be fine. This would help places like San Francisco and Washington DC where the subway lines are "subway" downtown, but more like a regional commuter rail out in the suburbs. But zoom 7 would be too noisy in places like Paris. Introducing light rail and tram at zoom 9 still seems good.

[nvkelso]

Connects with: #471 for national / regional rail zoom changes.

[nvkelso]

Related: #506

[nvkelso]

Subways are looking sharp at 8!

Light rail & tram are looking sharp at 9, too :)