mod_mailinglist: Do not crash when recipientdetails filter encounters an empty body
core: Getting unit tests running again

The testsandbox site no longer has a database.
core: Re-enable zotonic_status, fix build

Makefile changes were a bit over-optimistic.
core: Add testsandbox config file
tests: test master only for now on Travis CI; no database.
tests: Disable z_lib tag test

These tests broke on test systems with a timezone different than GMT+2
and were not generally very useful tests anyway.