Allow specifying Google Maps/Places API key via environment variable

[Holzhaus]

For the unsplash access key this is already possible. It makes sense to allow this for the Google API key as well.

[mauriceboe]

Thanks @Holzhaus. This is in #2500 and will ship with the next big release: set PLACES_API_KEY to your Google Maps key and it takes priority over the one in Admin → Settings → API Keys, for every member, the same way UNSPLASH_ACCESS_KEY works. The Google field in the admin panel then turns read-only and names the variable, and Test still checks the key. On Helm it goes under secretEnv.

[mauriceboe]

As mentioned above, PLACES_API_KEY is in and ships with the next big release. Closing, thanks @Holzhaus!