Display @-wildcard in user mention autocomplete based on the related policy

[sm-sayedi]

With #889, we will have a user-mention autocomplete with @-wildcard options available, such as @**channel**, @**topic**, and similar. These options are shown to all users unconditionally, but in fact, there's a realm-level policy (realm_wildcard_mention_policy) for who can use these wildcard mentions. We should display these @-wildcard mention options based on this policy in the Channel and Topic narrows.

Related links:

• https://chat.zulip.org/help/restrict-wildcard-mentions
• Search for realm_wildcard_mention_policy in https://zulip.com/api/register-queue

[gnprice]

Thanks for filing this!

Set to the M7 Future milestone, because the legacy zulip-mobile app doesn't pay attention to this permission either (and I don't think we've ever heard user feedback about it).