Add a gibberish mode (closes WFU-4407)

Test test:

  - update your config/settings.yml
  - set i18n_gibberish=1
  - load a page and see that all lower case translated text is replaced by '*'
  - set i18n_gibberish=0
  - check that everything works normally

There are three pull requests in this batch:

https://github.com/Work4Labs/w4l_symfony/pull/10
https://github.com/Work4Labs/work4core/pull/15
https://github.com/Work4Labs/work4us/pull/3574