Fix Google maps overflowing viewport

This adds a (somewhat hidden) feature to Kanisa-flavoured
markdown, where you can add a map by using:

{gmap:[msid]}

where msid can be obtained by finding the MSID parameter
from a Google maps URL. Since it seems not all maps have an
MSID, I'll probably change this in future to just be a
widget that takes a link to a map, and configures it for
embedding.
We should stop leaking minified files with every deployment
Add a couple of niggling mobile layout issue TODOs
Use chosen for the sermon form for selecting speakers and series
flake8 fix