2.0 migration (meta issue)

[danielmewes]

Horizon 2.0 changes the metadata format.

• hz migrate to migrate existing Horizon metadata in a RethinkDB server hz migrate for 2.0 #705
• hz schema save and hz schema apply should switch to a new structured format to specify indexes in the schema.toml files New index specification format for hz schema #706
• hz schema apply needs to detect old index definitions and interpret them, so that users can still import existing schema files Backwards-compatibility for hz schema apply #707
• The Horizon server should detect existing Horizon metadata in RethinkDB and error out, suggesting migration (hz migrate) Server: detect outdated metadata #708
• Documentation Horizon 2.0 docs checklist horizon-docs#104