Concurrent editors of the same document should be aware of which blocs are being edited

Solution:

Adding a `beingEditedBy` attribute to the block and using the same activity stream as #250.

#### Phase 1
- [x] Scenarios (Gherkin)
- [x] Mockups
- [x] Implementation strategy
#### Phase 2
- [ ] Acceptance tests (Capybara)
- [ ] Implementation
### Phase 3
- [ ] Screencast