ArchiveIt "frozen_only" & other bugfixes

[Fasand]

RE #534

• get_seeds now accepts an argument frozen_only, which preemptively returns an empty set if True and seeds haven't been frozen yet. This is so that old not frozen Harvests don't recursively slow down get_previously_harvested
• Fixed bug in "get seeds by frequency" where "0-frequency" seeds were being included – these are now only included in get_oneshot_seeds under oneshot conditions (not previously harvested, ...)
• When editing a Harvest, don't auto-select "ArchiveIt" when it wasn't selected previously. When changing harvest type or creating a new Harvest, the behaviour is unchanged – ArchiveIt is checked by default.