habit cards

[Seerden]

Closes #109. Closes #110.

I figured there's no point in implementing a mocked UI if the next step is making the mocks functional, so I just did it all in this one PR.

[Seerden]

How it's looking:

• [ ] need to figure out the deal with having entry_ids and entries on the same habitWithEntries object. entry_ids is only the real entries obviously, and I don't see the point in removing the field just because it "looks" cleaner, but still, having the two fields there simultaneously is weird.
• [ ] since we're invalidating the habit-entries cache on entry mutation, does the completion instance get updated right away too? Otherwise, we need to make sure we remove the 'synthetic', I think. Or maybe not, maybe it doesn't matter if the UI thinks it's still functional. Maybe it does, though. Don't want to create duplicates accidentally.