edits to a deletable garden/modal app should reset their TTL

[OwenPriceSkelly]

follow up to #327

A surprising case we'd like to avoid is one where a user creates a draft garden with a modal app, then later adds functions from that app to a new draft garden. Even though the second garden is brand-new, the app might get deleted and its functions would disappear without the user knowing why.

any edits to a deletable entity or new relationships between them should reset their expiration date.

[hholb]

For extra context: the TTL for a garden or modal app is tracked in the marked_for_deletion field. The field is nullable, so a reset could be as easy as garden.marked_for_deletion = None.