Because manually editing the URL isn’t a great user interface, this
commit adds a search field to do this on the user’s behalf.
For this pass at the story it doesn’t do any validation – the user will
just get no results if they search by something which isn’t a phone
number or email address.
If the user navigates to a different ‘bucket’ of notifications (eg
delivered, failed) then the search term is reset, because they’ve
changed the filter which is at a level above the search term.