Display HTML5 notifications when a task is no longer running

[hoelzro]

Sometimes bringing up a set of machines can take a while, and the user would like to know when the process is complete. Currently, the user has to check back with Asgard every so often. This commit introduces the use of HTML5 notifications (which are not used if unsupported by the user's browser) to inform the user when the task has completed or failed.

[cloudbees-pull-request-builder]

asgard-pull-requests #472 SUCCESS This pull request looks good