feat(workflow): add initial state/action registration

introduce `superdesk.workflow` module with `workflow_state` and
`workflow_action` registers

SD-1213 SD-1209
test(workflow): improve get action by state test case
Merge pull request #421 from petrjasek/feat-workflow

feat(workflow): add initial state/action registration
fix(celery-beat): don't use pidfile; custom scheduledb path
chore(readme): updated 'fig' section
chore(fig): added missing env vars
[SD-1214] Instances of actions for states
[SD-1214] Instances of actions for states
[SD-1220] Create missing privileges
[SD-1217] Backend: All ingested items will have the state ingested
[SD-1220] Create missing privileges - improvements
Merge remote-tracking branch 'upstream/feat-workflow' into feat-workflow
[SD-1220] Wired up missing privileges
fix(ingest): make sure we ingest content as str not bytes

this will prevent errors with replacement rules which don't work
on bytes content being unicode

SD-703 SD-1199
Merge remote-tracking branch 'upstream/feat-workflow' into feat-workflow
[SD-1216] Backend: Implement that actions update the state. Implemented 'fetched' state
Merge remote-tracking branch 'upstream/feat-workflow' into feat-workflow
[SD-1225] Refactor spike to use state
Fixing as per PR comments.
[SD-1225] use config value for state