Switch to using 'rails test:all' in documentation and CI Fixes #3116. This allows us to drop our custom patch for running system tests, as well as opening up all the usual rails test goodies like running individual files or directories.
Locale maintenance: * Rename be-TARASK to be-Tarask * Rename sr-EC to sr * Rename sr-EL to sr-Latn * Update README file in locales/ * Update languages.yml
Adding a README to the translations folder, so that people hopefully don't edit in there when they should be using TranslateWiki.