MM-22845: Added support for permalink previews. (#17796)

* MM-22845: Added support for permalink previews.

* MM-22845: Adds license to new file.

* MM-22845: Adds endpoint to retrieve multiple posts by id.

* MM-22845: Fix for deleted post.

* MM-22845: Adds config setting for permalink previews.

* MM-22845: Adds API test for new endpoint.

* MM-22845: Fix typo.

* MM-22845: Tests that post create or updated via App get the previewed_post prop.

* MM-22845: Tests for matching permalinks.

* MM-22845: Adds PreparePostForClient test for permalink previews.

* MM-22845: Embeds entire post in permalink metadata.

* MM-22845: Filter WS message payload of created and edited post based on permissions.

* MM-22845: Runs app layer generator.

* MM-22845: Lint check fix.

* MM-22845: Adds feature flag.

* MM-22845: Clones WS message.

* MM-22845: Removes knowledge of permalink from LinkMetadata table. Removes knowledge of user id from post embedding methods in favour of a 'sanitize' method/step.

* MM-22845: Handle nil post metadata.

* MM-22845: Switch to cloning post.

* MM-22845: Removes unused code.

* MM-22845: Refactor.

* MM-22845: Reverts whitespace change.

* MM-22845: Removes unnecessary code.

* MM-22845: Removes unnecessary function.

* MM-22845: Warn but don't error if permalinked referenced post or channel is not found.

* MM-22845: Fix for clone method.

* MM-22845: Fix for clone method.

* MM-22845: Updates translations.

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/message_export.go

@@ -3,6 +3,8 @@
 
 package model
 
+import "encoding/json"
+
 type MessageExport struct {
 	TeamId          *string
 	TeamName        *string
@@ -34,3 +36,15 @@ type MessageExportCursor struct {
 	LastPostUpdateAt int64
 	LastPostId       string
 }
+
+// PreviewID returns the value of the post's previewed_post prop, if present, or an empty string.
+func (m *MessageExport) PreviewID() string {
+	var previewID string
+	props := map[string]interface{}{}
+	if m.PostProps != nil && json.Unmarshal([]byte(*m.PostProps), &props) == nil {
+		if val, ok := props[PostPropsPreviewedPost]; ok {
+			previewID = val.(string)
+		}
+	}
+	return previewID
+}