group 50 services per schedule task for webshot

Rather than overload a single webshot process less split them into
groups of 50 and let the user run multiple scheduler services to handle
multiple processes.
fix ability to stop multiple tasks in the index

jQuery in tasks/index only picked up the first "#stop_button" for some
reason. Thus only the first button actually did anything. Changing this
to use $(".btn-danger") fixes it as long as we don't use more than 1
btn-danger on this page.
Tweaks to webimaging

- allow useragent to be set from args[3]
- default user agent includes "Kvasir"
- need to add ability in UI to select/enter a UA still