Simple job management page

Also added a seperate `job-indicator` for every job state (pending/awaiting retry and failed)
The jobs page is automatically reloading when jobs are enqueued, retried or failed.

For people running on the free heroku account the number of failed jobs might become an issue and even if you run huginn on your own server you might not want to have a few thousand failed jobs in the database. We could easily add a scheduler to keep the number of failed jobs down, but I wasn't sure what would be the best solution. I am tending towards adding a general `ENV` variable which defaults to 500.
`failed_at` will be set after the job errored for the configured x times, before it is `NULL`. 
I did not go for `delete_all` because it wasn't sure if DJ uses `after_destroy` callback, but I will check that and change it if possible.