Handle flags for @-topic mentions

[gnprice]

We should handle the topic_wildcard_mentioned per-user flag on messages:
https://zulip.com/api/update-message-flags#available-flags
https://chat.zulip.org/#narrow/stream/378-api-design/topic/highlight.20topic-wildcard.20mention/near/1621613

As part of the same PR, we should handle the also-new stream_wildcard_mentioned flag, and then make our handling of the now-deprecated wildcard_mentioned flag occur only as a fallback on servers that don't support the new flags. (In particular this means there should be // TODO(server-8) comments for us to delete the wildcard_mentioned code, leaving only the code for the new pair of flags.)

I believe the behavior we'll currently want is the same on either of the new flags as it is on the old flag. The one way I'm aware of that we'll eventually use the distinction is for #647, which is in the future and out of scope for this issue.