Sign up issues with phone-only (0.5d)s around alerts/updates

[dracos]

If a user is logged in with a mobile phone-only account, there's a few issues:

• get updates form on a report page still says "Receive email when updates are left on this problem" but if they click the button they'll be signed up to update notifications by text;
• Confirmation page after that says "Email alert created" or similar
• Alert signup page for a location says "Subscribe by email" with a button - but clicking it will sign you up but we don't currently do alerts by text, only report updates, so it needs to ask you for email/confirmation email as for a logged out user

Those seem the major places, may have missed more.

[dracos]

Fixed by #3367