Basic building blocks for workflows

From the milestone 2 requirements:

MUST: Implement basic building blocks for “workflows”: sequences/set of tasks, possibly building on each other, triggered by a single event.
  Available workflows are defined by the workspace administrators
  Workflows can be triggered through the API

We need to do some more investigation before we know the general approach we're going to take here, so this is likely to spawn several other issues. (I'm currently experimenting with a prototype of doing server-side actions via WorkRequests as well as the existing worker-side tasks; if that works nicely then we can avoid an additional layer of abstraction between work requests and workflows, which will be helpful.)