The default frequency is always single email

After testing "Daily Digest" option last night have a couple observations:

- "Daily Digest" e-mails were blasted out at 11:38pm EST
- They look exactly like "Single Emails", plus there is no digest, they're sent individually (all at once) just like Single Emails
- If keeping "Daily Digest," there needs to be unique email settings for the subject line and body
- If keeping "Daily Digest" it might be worth doing it for ALL forum topic and comment subscriptions, one email total for everything you're subscribed to
- In that same respect, "Single Email" would be a single option that would apply to ALL subscriptions. Instead of one-by-one, as it is now for forum topics and comments.

In our experience, Drupal Forum is not used much to begin with. So Daily Digest may be overkill and just make this module Single Emails. That will also simplify the module, as users will not need to choose.

This is fixed in this duplicate issue Invalid default frequency settings 🐛 Invalid default frequency settings Active