Allow users to request digest, even when no activity as occurred.

For me, the use case was:
- Client was having email issues, and distrusted the 'notifications' produced by gravity forms.
- They wanted to receive a regular digest (instead), even when there were no new leads captured.
- This is controlled via a checkbox in the digest notfication settings.

Additionally, addressed part of: https://github.com/soulseekah/Gravity-Forms-Digest-Bulk-Reports/issues/1
'Notifications digest' is now available from the main form menu