YAML is just too easy to get wrong

closes #8132

The quiet_assets gem doesn't support rails 5, but sprockets 3 now has a
config.assets.quiet option, which deprecates the quiet_assets gem. But
the logging gem has a no-op silence method, that's why the quiet option
doesn't work out-of-the-box. I added a little hack to use the silence
method from ActiveSupport, which is also used from the original rails
logger.

by default both are disabled now, you can enable them in the
diaspora.yml if you need them.

closes #6090

closes #5994

closes #5988