Workflows: allow a trigger to name several content types and several events

Found while planning barakocms.com as a barakoCMS-backed site.

`WorkflowDefinition` has one `TriggerContentType` and one `TriggerEvent`. To say "rebuild the site whenever anything changes in any of these three types" takes five separate definitions that differ only in those two fields and share one action.

Five copies of one intent drift. Add a fourth content type and it is four edits in four places, and nothing fails if you miss one.

## What to change

Let a trigger name several content types and several events. A list in each field, with the existing single value staying valid, so nothing already configured breaks.

Start by locating WorkflowDefinition and the code that consumes TriggerContentType and TriggerEvent. Trace how existing single values are loaded and matched, then verify that old configurations remain valid and that multiple content types and events can share one action.

Written by the indexing model from the issue text.