retention strategies. fixes #315
listing closed indices, reopen closed indices

Graylog2/graylog2-web-interface#401
do not run retention against reopened indices. Graylog2/graylog2-web-interface#401
marking reopened indices as reopened. Graylog2/graylog2-web-interface#401
delete index API

Graylog2/graylog2-web-interface#401
Merge branch '020' into authtokens