Net cdf download email address 2

Sorry for the big PR :-(  I got a bit carried away.

If I was going to do this one again, I probably would try to submit separate PRs for:
* refactor URL construction methods to all have same signature (so that they can all be used as a callback)
* extract common code between `NcwmsInjector` and `WmsInjector`
* introduce `DownloadPanel` (but not necessarily visible/used)
* remove email text field from download panel, start using the one in confirmation popup.