Migrated subscriptions list screen to Compose

[ArnyminerZ]

Depends on #160

[github-actions]

This PR/issue depends on: * #160

[ArnyminerZ]

Okay, I think now everything is ready

[rfc2822]

Thanks, I tried to accomodate and have moved things around a bit.

• Settings moved from ui package to main package to distinguish between logic (Settings) and UI (may call Settings to read/change them)
• MaterialAlertDialogBuilder saves us from implementing the sync interval dialog ourselves.
• Add TODOs to replace Snackbar by real warning box; again move logic (like calculations when to ask for a permission) to model

[ArnyminerZ]

Maybe for permissions we can use Accompanist. I've used it, and it works like a charm.

[ArnyminerZ]

I've moved the warnings from snackbar to some little cards:

The issue now is that the "no subscriptions" text is a bit strange with all of this (see screenshot above).

It's technically aligned below the cards, but when there are no cards, it shows up like this:

I think we should simply not show this message, and even hide the add FAB when the calendar permission is not granted, since synchronization will never run without this. However, maybe we want to allow the user to be able to add subscriptions before granting permissions, if that has any sense, I don't know. Whatever the case, we still have that issue when showing the notification and battery optimization cards. I've thought about aligning it "as a list item" when there are cards, and center it otherwise. But it's open for new ideas.

What do you think? @rfc2822 @devvv4ever

[rfc2822]

However, maybe we want to allow the user to be able to add subscriptions before granting permissions, if that has any sense, I don't know.

That was the idea. Let's continue rewriting to Compose and then we can still optimize the experience.