finished indexer failures integration. fixes #641
documented index failures. fixes #643
show informative error when dashboard has been deleted but was chosen as a startpage

fixes #645
more information in indexer failures table. #653
Merge branch '0.20' of github.com:Graylog2/graylog2-web-interface into 0.20
make DeflectorExistsAsIndexNotification closable. Graylog2/graylog2-server#452
updated documentation links for dead letters. #653
bump to 0.20.1
Merge git://github.com/Graylog2/graylog2-web-interface into develop