Exclude restricted accounts from quota usage notification emails

[adamalton]

This is a follow-up to #990 (and so is dependent on it).

When we email users telling them that they are approaching or exceeding their storage quota, we should exclude accounts which are restricted (by checking the user_tags table).

The rationale is that if a user's account is restricted then there's no point in warning them about their usage level and that their account might become restricted. When their account becomes restricted that should trigger a separate email telling them about that (which is a separate task, not tied to this), and these warning emails are no longer relevant.