Don't escape new line in Etherpad welcome message

Summary:
Both the json (Salt) and tojson (Jinja2) filters escape new lines
as \\n in JSON strings.

In this specific case, we need a direct \n, as we output configuration
and not a string template to be rendered aftewards.

This new line escape dance is a little bit hacky, but it's not straightforward
to determine what kind of JSON it needs to produce, to create a new filter.

This is a follow-up for dda0e01728ce.

Ref T1871.

Test Plan: Restarted Etherpad, created new pad, new lines are OK

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1871

Differential Revision: https://devcentral.nasqueron.org/D3106