Simplify the default configuration file

After some discussion with the community it was decided to
clean the configuration file from excessive comments and
explicitly configured default values. Also, mod_mam and
mod_http_upload have been added.

The rationale for this is to have a clean and not bloated
configuration file which doesn't scare away newcomers and
which has all features from the Compliance Suite 2018 (XEP-0387)
enabled by default.

For further configuration an admin is encouraged to read the
documentation at https://docs.ejabberd.im/admin/configuration