inbox: Make channel folders collapsible #2220

In #2186, we'll start showing channel folders in the inbox. They won't be collapsible, though; this issue is to make them collapsible.

See suggestion in Figma: https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=13188-51969&m=dev

Related issues:

• inbox: Show unread counts and "@" on channel-folder headers, when collapsed #2259