Change wording of form hints on register page

We’ve seen in research that people can be reticent to give their real
phone number. Telling them that it will be used for something should
help (ie we’re not just collecting it for marketing).

This also rewords the other form hints on this page to be less computery
because we haven’t looked at them in aaaages.

app/main/forms.py

@@ -36,11 +36,11 @@ class UKMobileNumber(TelField):
 
 
 def mobile_number():
-    return UKMobileNumber('Mobile phone number',
+    return UKMobileNumber('Mobile number',
                           validators=[DataRequired(message='Can’t be empty')])
 
 
-def password(label='Create a password'):
+def password(label='Password'):
     return PasswordField(label,
                          validators=[DataRequired(message='Can’t be empty'),
                                      Length(10, 255, message='Must be at least 10 characters'),