WIP: delete modal dialog

Note: this PR depends on !107 (merged)

This PR transforms the job deletion confirmation page into a modal dialog in the JobList page and sends POST /jobs/delete request in ajax. This make the deletion much more convenient because there is 0 page loads instead of 2.

Although this is working, I am not satisfied with the implementation because it is a little too low-level (with dirty javascript), especially since i know how to implement the same thing in rails with less javascript (the button_to helper with remote: true makes sending ajax requests very easy, perhaps there is something equivalent in django). Also i do not know how to report ajax errors to the user in a clean and consistent manner.

So I am letting the PR there for the record. I think this should be investigated together with other UI improvements (things like client-side form validation¸ turbolinks, ...).