channels: show view count

[r10s]

when a core with the new api get_message_read_receipt_count() is released, we want to show the view count for channel owners, as folllows:

the view count should be shown unconditionally, so also if 0.

the ticks shall not be shown, only pending/error state.

in the chatlist, neither view count nor ticks are shown, only pending/error state

counterpart of deltachat/deltachat-android#4176 and deltachat/deltachat-ios#2990