Consistently set file encoding to utf-8

I've left out script/rails and script/cucumber but have added the encoding to
every other file.
Prefer single-quoted strings

... when you don't need string interpolation or special symbols.
Remove trailing whitespace
Prefer lambda literals
Add missing spaces
Remove unnecessary parentheses
Remove non-ascii characters from comments
Avoid 'Ambiguous first argument' warnings

I've left one instance of this warning in place: line 17 of
statement_importer_test.rb because I think it's more readable in its current
form (`assert_equal -123, <actual-value>`).
Use MatchData instead of $1

I'm still getting these warnings from paths.rb and selectors.rb in
features/support but it's not obvious how to change those, and I'm hoping to
remove them soon(ish).
Remove unused variables
Merge branch 'rubocop-suggestions'

I've fixed most of the warnings generated by rubocop but do have a few left.

Those left are either:

* 'Line is too long', which I'm not going to attempt to 'fix' in bulk

* In the auto-generated db/schema.rb which there's nothing I can do about

* In the feature step definitions that I'm planning to change when I change
the style of Cucumber in use in the project.