Add "Uncheck All" link to bulk actions form

[juliancbrown]

If a user has selected tickets it might be difficult to dismiss the bulk action form. Currently dismissing the form requires either:

1. individually unchecking all selected rows
2. Checking then uncheck the table header checkbox

These methods aren't readily discoverable and depend on the users current scroll depth.
Adding a link would make the UX for dismissing this form simpler. I'd propose placing this immediately after the bug count:

[julianandrews]

I've pushed changes on the 'uncheck-all' branch. It'll need some restyling!