Allow rebuild of aborted builds

- The "rebuild all" button will now rebuild build attempts that were aborted. Fixes #21 
- The "rebuild all" button will be displayed if every build part is in aborted state
- Change the semantics of the `request_build` action some. If a build is request for a branch that doesn't have a new commit to build, the user will be redirected to the page for the latest build.

To: @cheister