MM-42810: Using websocket broadcast hook for permalink preview (#28627)

We use the newly introduced websocket broadcast hook system
to implement permalink preview efficiently. This is essentially
a re-do of https://github.com/mattermost/mattermost/pull/23812
using the new system.

https://mattermost.atlassian.net/browse/MM-42810
```release-note
NONE
```

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/platform/helper_test.go

@@ -57,6 +57,9 @@ func (ms *mockSuite) RolesGrantPermission(roleNames []string, permissionId strin
 func (ms *mockSuite) UserCanSeeOtherUser(c request.CTX, userID string, otherUserId string) (bool, *model.AppError) {
 	return true, nil
 }
+func (ms *mockSuite) HasPermissionToReadChannel(c request.CTX, userID string, channel *model.Channel) bool {
+	return true
+}
 
 func Setup(tb testing.TB, options ...Option) *TestHelper {
 	if testing.Short() {