TMSS-190: let stoptime be set by client(s) instead of assuming that a state update to finishing should always set the subtaks's stoptime

Resolve TMSS-421

Closes TMSS-421

See merge request !262

TMSS-322: reimplemented this feature. As stated in the ticket: such a...

Closes TMSS-322

See merge request !267

TMSS-190: and TMSS-421: merge TMSS-421 into this feature branch. Added a radbpglistener to the ratestenvironment.

TMSS-421: processed review comments. adapted radbpglistener add_notifications and handle the change in the payload (from a single id to a jsonblob)

TMSS-190: removed obsolete method calculate_start_time. Added SubtaskSchedulingException when the duration is less then 1 second.

TMSS-322: reimplemented this feature. As stated in the ticket: such a convenience log_url is not part of the Subtask model, but can be a view on the model. So, implemeted it in the viewset. Now, fetching subtasks from the TMSS db is superfast again.

TMSS-190: and TMSS-388: only enable viewflow if TMSS_ENABLE_VIEWFLOW envvar is set, or if --viewflow option is given

Resolve TMSS-422

Closes TMSS-422

See merge request !266

Resolve TMSS-403

Closes TMSS-403

See merge request !257