Unify JobStatus and RunStatus enums across components

[VMois]

Currently, JobStatus (enumeration of possible job statuses) and RunStatus (enumeration of possible workflow statuses) reside in reana-db repo. But, the values that represent job and workflow statuses are used by workflow-engines and, as of now, are duplicated in each of them. The engines do not depend on reana-db so they cannot import those enums.

What do you think if we move JobStatus and RunStatus enums from reana-db to reana-commons repo? How will it affect future maintenance of engines, reana-db and reana-commons repositories?

Originated in this PR

[VMois]

In addition, r-job-controller is returning started JobStatus which is not listed in reana-db, reanahub/reana-job-controller#337.

[VMois]

Related issue reanahub/reana-workflow-controller/issues/204 which lists adding enums to r-commons as a possible solution.