Use member count instead of counting members (#530)

Use member count instead of counting members

For the room details screen, use the member count as supplied by
matrix instead of waiting for the entire member list to be
retrieved and then manually adding up all the relevant users.

This removes the loading state of the member count, relying on
a spinner on the member list itself if the user actually wants
to see the members. (The performance of that will be improved
separately on the rust side in the future)

Closes #505

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