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() {

server/channels/app/platform/mocks/SuiteIFace.go

@@ -48,6 +48,24 @@ func (_m *SuiteIFace) GetSession(token string) (*model.Session, *model.AppError)
 	return r0, r1
 }
 
+// HasPermissionToReadChannel provides a mock function with given fields: c, userID, channel
+func (_m *SuiteIFace) HasPermissionToReadChannel(c request.CTX, userID string, channel *model.Channel) bool {
+	ret := _m.Called(c, userID, channel)
+
+	if len(ret) == 0 {
+		panic("no return value specified for HasPermissionToReadChannel")
+	}
+
+	var r0 bool
+	if rf, ok := ret.Get(0).(func(request.CTX, string, *model.Channel) bool); ok {
+		r0 = rf(c, userID, channel)
+	} else {
+		r0 = ret.Get(0).(bool)
+	}
+
+	return r0
+}
+
 // RolesGrantPermission provides a mock function with given fields: roleNames, permissionId
 func (_m *SuiteIFace) RolesGrantPermission(roleNames []string, permissionId string) bool {
 	ret := _m.Called(roleNames, permissionId)

server/channels/app/platform/web_hub.go

@@ -24,6 +24,7 @@ const (
 type SuiteIFace interface {
 	GetSession(token string) (*model.Session, *model.AppError)
 	RolesGrantPermission(roleNames []string, permissionId string) bool
+	HasPermissionToReadChannel(c request.CTX, userID string, channel *model.Channel) bool
 	UserCanSeeOtherUser(c request.CTX, userID string, otherUserId string) (bool, *model.AppError)
 }