Better list item support

Breaking:
- To edit list item notes with `ListItem.edit`, you must now pass `note_content` or use `ListItemNote.edit` (`listitem.note.edit`).
- `ListItem.has_note` is no longer a property and is instead now a function that must be called.
- `ListItem` no longer has direct attributes for note properties. They must now be accessed via `ListItem.note`, e.g. `listitem.note_created_at` -> `listitem.note.created_at`.
- `ListItem.fetch_parent` now raises `ValueError` if the item has no parent instead of falling back to an `HTTPException`.
Changed:
- `ListItem.note` is no longer optional. Use `ListItem.has_note()` to determine whether an item has a note.
- Improved signatures for `ListChannel.create_item`, `ListItem.create_item`, `ListItem.edit`, `ListItemNote.edit`
- Removed redundancy in some docstrings
Added:
- `ListItem` mentions data for bot accounts
  - `ListItem.assigned_to` for bot accounts
- `ListItemNote` mentions data for bot accounts
- `ListItem.complete()` and `ListItem.uncomplete()` for bot accounts

Related to #34