Enables optional access to Etherpad's web-UI. This is useful for
managing Etherpad plugins.

Among other things, plugins makes it easy to manage/delete pads if you
install the adminpads2 plugin.

Extend the config template with the lines added in the past year

Signed-off-by: Ahmad Haghighi <haghighi@fedoraproject.org>

This reverts commit fd3d48bb6d.

Normally this environment variable gets referred to from `settings.json.docker`,
but we have our own full configuration, which hardcodes `"trustProxy": true`,
thus making this useless.

This has been pointed out here:
fd3d48bb6d (commitcomment-47403097)

The official Etherpad Docker image has no support for sqlite3 databases.

don't bind to any host port if nginx_proxy is used
only bind to localhost if it's not used