Make bulk edit options clearer

* White checkboxes on white backgrounds are easily missed
* Bring attention to the checkboxes by darkening the background
Give checkbox column a header

* Label checkboxes with “Edit”, and include a tooltip about how to use
bulk editing tools
* Automatically start all tooltips and remove an old inline script
Tooltip if hovering on disabled bulk edit buttons

* Users often try to interact with the disabled bulk edit buttons
* Include a tooltip when hovering on the buttons whilst they are
disabled, with an explanation of how to use the buttons
* When a selection has been made the tooltip won’t show
Remove selectable table script call in style guide

* We don’t instantiate like this any more
Include a border on the edit mapping header
Merge pull request #252 from alphagov/edit-mapping-tip

Highlight bulk edit tools and include guidance