Grow Enquiries 0.2.0

Phase 2 of the enquiry module plan.

• Digest of held spam: an email listing what the spam filter held since the last one, and any genuine enquiry whose email failed, each linking to its enquiry. Monthly (the 1st) by default, or weekly (Mondays), or off; to the recipients or a separate address (Settings, Retention and digest). Only sent when there's something in it. "Send a preview of the digest" on the Tools tab, and wp grow-enquiries digest.
• Failed emails retried hourly, up to 3 times, then flagged: a red "Not emailed: reply by hand" badge, a notice on the Enquiries screens, and a "Not emailed" view (shown when there are any) with a bulk Resend email. The enquiry view shows the mail error and how many retries are left.
• CSV export of the view on screen (Inbox, Spam, Not emailed or Bin), with From and To dates that also filter the list. Cells that a spreadsheet would run as formulas are made safe.
• Personal data tools: Tools, Export Personal Data and Erase Personal Data find enquiries by email address. Suggested wording added to Settings, Privacy, Policy guide, following this site's retention settings (also shown on the Retention and digest tab).
• Dashboard widget: unread, held as spam this month, and not emailed.
• Updates from GitHub releases: bundles Plugin Update Checker 5.7 (MIT), pointed at aconitewill/grow-enquiries; sites see new versions in Dashboard, Updates. Off in a git checkout (so updating a dev copy can't wipe .git) or with GRE_DISABLE_UPDATES. A GitHub Action builds a clean zip on each v* tag and attaches it to the release.
• Fix: gre_do_action() and gre_action_url() now load everywhere, not just in wp-admin, so WP-CLI and scripts can release or mark enquiries. New wp grow-enquiries command: list, release, spam, read, resend, bin, retry, digest.