We are using `can_add_subscribers_group` instead of
`invite_to_stream_policy` to check whether a user can subscribe other
users.
We've removed `invite_to_stream_policy` from the frontend wherever
applicable.
Previously, the number of subscribers was displayed on channel pills.
This commit removes the subscriber count from the channel pills and
adds it to the channel cards. The count is styled similarly to the
number of group members in group cards, with the text 'N subscribers'
(or '1 subscriber' for single subscribers).
Fixes: #32235.
