Consider making pending status transparent to the end user

[audrium]

Stems from #363

pending status is very similar to queued and could be considered as internal and transparent to the user. Meaning that user using CLI or UI would not strongly benefit by seeing it or even could be confused by the difference of queued and pending status.

Two potential ways to hide it would be:

• Create a new hybrid property in reana-db (e.g. workflow.status_ui). More info here. Caveat is that there would be two similar properties (e.g. workflow.status and workflow.status_ui) which would be confusing for developers and could lead to potential errors.
• Handle it in reana-workflow-controller by returning queued in case the status is pending. Caveat is filtering by status. For example in the UI we allow to specify statuses to filter by, so in queued case we should also include pending here.