Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

As per: https://tools.ietf.org/html/rfc5737

Fixes that make this work:

* A change in Ruby (https://github.com/ruby/ruby/commit/ce635262f53b760284d56bb1027baebaaec175d1)
requires passing in the exact required length for OpenSSL keys and IVs.

* Ensure the secrets.yml is generated before any prepended modules are
loaded. This is done by renaming the `secret_token.rb` initializer to
`01_secret_token.rb`, which is a bit ugly but involves the least impact on
other files.

Updated documentation as per latest missing features on about.gitlab.com

See merge request gitlab-org/gitlab-ee!5794

Signed-off-by: Rémy Coutable <remy@rymai.me>