Use single quotes everywhere possible in our code

ie everywhere except where we're using string interpolation, or raw
JSON, or an occasional spec description which includes quotes.

* WARNING: personal opinions may be present*

I find single quotes easier to read in general, and like to be able to
see at a glance from the quotes whether or not a particular string uses
interpolation. We've been following this rule in Transition for some time,
only using double quotes where necessary.

This does contradict our style guide, which comes down in favour of
double quotes, or at least not using both near each other:

    https://github.com/alphagov/styleguides/blob/f88150845037cae27f5c23d666e43f1081fc96f7/ruby.md#strings