Fix #467: pass the Google Maps API key to the schedule viewer.

[andrew-hardin]

This is a straightforward fix that resolves issue #467 by allowing the GMaps API key to be configured by the Python server.

A related note: the default API key registered to gtfs.schedule.viewer@gmail.com seems to still be valid 👍

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[andrew-hardin]

I signed it!

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[amenk]

@andrew-hardin The CI fails, do you know why?

[andrew-hardin]

@amenk: not sure why it failed, but I merged the branch with a recent copy of master and the build passed.

[answerquest]

Hi, is this good to go?