The _default_ mapping deprecation/removal

[lukas-vlcek]

The _default_ mapping is deprecated in ES 6 and removed in ES 7.

See:

• https://www.elastic.co/guide/en/elasticsearch/reference/6.0/default-mapping.html
• https://www.elastic.co/guide/en/elasticsearch/reference/6.0/removal-of-types.html#_schedule_for_removal_of_mapping_types

[lukas-vlcek]

I propose to replace index type names including the _default_ one in index templates as suggested in #84 (comment).

[lukas-vlcek]

Implemented in 0e3ccaf except the commit message should have been probably formatted differently for the GitHub to correctly recognise the issues associated with "Closes" string (no comma separated issues supported?).