closes #7570

It looks like all commands we use in production fixed the warnings
(rails 5.1), so we can update that again.

Needed for `assigns` in controller specs

This commit introduces changes to the user data export archive format.
This extends data set which is included in the archive. This data can be
then imported to other pods when this feature is implemented.

Also the commit adds the archive format json schema. ATM it is used in
automatic tests only, but in future it will also be used to validate
incoming archives.

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.