Maintain cursor pointer on inputs

* When hovering on the checkboxes in selectable rows the cursor would
change from the pointer to the default arrow, which is a little
confusing
Clarify the meaning of bulk edit buttons

* Use title attributes to explain what will happen for each button and
the checkbox
* Remove the ambiguous “selected” text, limit buttons to a single word
verb
Include a count of the mappings being bulk edited

* Be clear about the number of things being selected, eg all from
current page, not total
* Allows the user to validate a little more easily
Merge pull request #168 from alphagov/bulk-edit-improvements

Minor bulk edit improvements