Code in the `boot/` module needs to be compiled to work in older
browsers than code shared between the annotator and sidebar applications
in `src/shared`. To ensure this it helps to avoid dependencies from
code in `src/boot` on code in `src/shared`, but the converse is OK.

This commit moves the `jsonConfigsFrom` function from `shared/settings`
to `boot/parse-json-config` so that it is compiled with the same
settings as the rest of the code in the `boot/` module and to reduce the
chances of someone accidentally introducing ES6+ features into this code
in future and forgetting about older browsers.

The function was also renamed to make it more obvious that it parses
JSON config, and thus may fail, rather than returning a JSON string.