add a check again link when upload validation times out (bug 957158)

https://bugzilla.mozilla.org/show_bug.cgi?id=957158

When uploading an addon that takes a long time to validate the connection can get killed because the connection to check validation waits for validation to complete. This adds a "Check again" link that will check if the validation has completed so that the user can continue with their submission.

![retry-failed-upload m4v](https://cloud.githubusercontent.com/assets/211578/2962109/710e5b7e-dacd-11e3-9607-f514ced97798.gif)
I had to add `import time; time.sleep(2); response.content = 'lol'` to apps/devhub/views.py:900 to make the request fail as described in the bug and remove the line to make it pass. This bug cannot be reproduced on dev or stage.