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

[chrisbobbe]

In #2186, we'll start showing channel folders in the inbox. The channel-folder headers won't have unread counts or "@", though; this issue is to add those.

Related issues:

• inbox: Make channel folders collapsible #2220

[chrisbobbe]

Added "when collapsed" to the title; see #mobile-design > channel folders in inbox: design @ 💬

where Vlad said:

adding their unread counts and "@" markers

I don't think such markers are needed for unfolded Groups. For folded groups it makes sense to have them.