Sanitize RemoteEmail user prop (#27170)

* Sanitize RemoteEmail user prop

If the server is configured to hide user emails, the "RemoteEmail"
user property will be sanitized as well, effectively hiding the real
email of remote users.

* fix merge conflict

---------

Co-authored-by: Doug Lauder <wiggin77@warpmail.net>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/platform/services/sharedchannel/util.go

@@ -18,7 +18,7 @@ func fixMention(post *model.Post, mentionMap model.UserMentionMap, user *model.U
 		return
 	}
 
-	realUsername, ok := user.GetProp(KeyRemoteUsername)
+	realUsername, ok := user.GetProp(model.UserPropsKeyRemoteUsername)
 	if !ok {
 		return
 	}