Use locale domain `messages` in gettext.js

Was hoping to use `gettext.js` in conjunction with grunt-i18n-abide.

This pull:
* removes the domain `client` and uses the more familiar `messages`
* synchronizes what is produced with `abidecompile` - [relevant source](https://github.com/mozilla/grunt-i18n-abide/blob/d5da93cb3ab31b5e95e5c17383afe09cd903b4ae/tasks/abidecompile.js#L72)
* fixes linting errors