Clean up eslint output, improve code quality

There are JavaScript tools (eslint) that help us enforce consistent coding style. More important, eslint also diagnoses some conditions that could lead to bugs, or to working-by-coincidence, that really ought to be fixed.

To some extent we can fix lint-errors in passing while working on other things. But because they often affect large regions of files (e.g. changing orders of functions, or moving methods to class functions) these changes don't play well with other work going on at the same time, and tend to lead to difficult merges. So it would be good to set aside some time to deal with these things in isolation.

Related to this (and arguably in the remit of STRIPES-5) is configuring eslint so that it applies the rules that we want, and not (all) the rules that AirBNB want. We like their general approach, but we'll need to tweak it.