Improve the whitelist view

- Add the ability to filter the whitelist table, like the organisations table.
- Add a Cucumber spec to test that only admins can access the whitelist.
- Highlight the row containing the domain just added, so it can be found easily and we can spot typos.
- Add the ability to sort the table by date added, in either ASC or DESC order, using the TableSorter jQuery plugin.

(Rebased today (17th June) against master.)
Huh. I must've left this page open then. If I scroll up github says the diff's outdated but it'll still let me comment on it without any warning. Usually it would collapse the diff :angry: