Ensure that changes to models propagate to the collection.

I'm need to look for cases where this might cause redundant saves, but I
know it's necessary in some cases, and fairly harmless even if there are
duplicates.
Provide some conveniences for dealing with new settings:

.getOrDefault(id, defaultValue): Get setting 'id' value, or return default.
.setOrCreate(id, value, retain): Set an existing setting, or establish it.
Replace redundant fetches of the form values with the already-fetched ones.
Use the new settings facilities to retain the last add-public-share form's
"Remember" state, within and across sessions.  Easy peasy!