make sources list time range configurable

fixes Graylog2/graylog2-web-interface#599
show local hostname in main layout footer. fixes #600
disallow creating dashboard widgets with streams that the user isn't allow to read

fixes a security hole if the attacker knows a stream id they aren't supposed to see (unlikely but was possible)

fixes Graylog2/graylog2-web-interface#594
Merge git://github.com/Graylog2/graylog2-web-interface into develop