Use an 'appendstack.Accumulator' for profile's '_pending_alerts.

Avoiding conflicts, both with other web requests and with digest script.
Update digest processing to use 'Accumulator.consume' API.
More explicit test, better clarity.
Add evolve44 to convert profiles to use Accumulator for _pending_alerts.
Add evolve44 to convert profiles to use Accumulator for _pending_alerts.
Note change for this branch.
Add 'weekly' and 'biweekly' digest options to community subscriptions.
Handle digest frequencies when processing digests.
Pass digest frequency on script command line.
Merge tseaver-1122345-deconflict_profile_pending_alerts.
Merge tseaver-1132888-new_digest_frequencies.
Spacing.
New digest periods weaken 'conflict-free' to 'conflict-resistant'.
Prep 3.108 release.