Subscription List Changes

Made some changes to the subscription list. See also the accompanying MlemMiddleware PR.

Moved the subscription list to the correct location. All of the list rows go to the Subscribed feed atm.
Added the alphabetic jump bar.
Avatars are now shown in the community list.
The position of the instance host can be changed in Settings. It defaults to .bottom on iPad and .trailing on iPhone.
Added the ability to group by instance rather than alphabetically.
Added "undo" toasts for unsubscribing and unfavoriting.
Changed the list style to .sidebar on iPad, which I think looks a lot cleaner.
Moved the "Favorite" action to the context menu, which leaves more room for the instance host to be displayed.

Closes #1108

mlemgroup / mlem