The caseworking view is going to have a page which displays emails and
text messages combined together.
In order for the search to work on this page the user needs to be able
to search for an email or a text message. This commit makes it guess
what to search for when the `notification_type` isn’t known (basically
by saying ‘if the search term is only digits they’re probably looking
searching by phone number’).
27faf7912b