Release 6.0

Closes caosdb/internal/all#650 and caosdb/internal/all#526

See merge request !17

External String IDs

See merge request !15

Merge branch 'f-external-ids' of gitlab.indiscale.com:caosdb/src/caosdb-mysqlbackend into f-external-ids

This reverts commit d7d9e6f6.

This reverts commit 6570e865.

For #34.

For #28

"trigger" keyword and failing strategy

Closes caosdb/internal/all#650 and caosdb/internal/all#526

See merge request !8