Hide map on index page if no API key. Fixes #879

[fbacall]

Summary of changes

• Hide map on index page if no API key.
• Tweak custom Config getters to be dynamic.

Motivation and context

#879

Setting derived TeSS::Config properties on boot (e.g. map_enabled) made it hard to switch when testing.

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.