Updating delete actions so that they use HTTP DELETE

This PR fixes #103.

Updates delete actions from list views so that they use HTTP DELETE (via `Backbone.Model.destroy()`).

TODO:

- [x] Add confirmation for the delete action
- [x] Add base item view and base model to reduce code duplication for delete actions
- [x] Update projects and client delete actions
- [x] ~~Re-fetch current listing content after deleting the model~~ (discussed with @markwilson - will live with this for now)

Off the back of this PR, we need to implement soft deletions (#110), and then at a later date add support for failed deletion attempts (I'd expect a voter to deny a delete action and we would hook into that).