[MM-42194] Get file information from a deleted post (#20279)

* Introduced inlcude_deleted query parameter to allow admins to retrieve contents of post regardless of deletion status

* Introduced new client route and tests for getting file info of deleted posts

* Fixed tests due to caching of posts

* gofmt

* Small formatting updates

* Invalidating file infos cache on delete of post if post includes files

* Including deleted in migration flow

* Moved invalidating of cache

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

model/client4.go

@@ -4419,6 +4419,24 @@ func (c *Client4) GetFileInfosForPost(postId string, etag string) ([]*FileInfo,
 	return list, BuildResponse(r), nil
 }
 
+// GetFileInfosForPost gets all the file info objects attached to a post, including deleted
+func (c *Client4) GetFileInfosForPostIncludeDeleted(postId string, etag string) ([]*FileInfo, *Response, error) {
+	r, err := c.DoAPIGet(c.postRoute(postId)+"/files/info"+"?include_deleted="+c.boolString(true), etag)
+	if err != nil {
+		return nil, BuildResponse(r), err
+	}
+	defer closeBody(r)
+
+	var list []*FileInfo
+	if r.StatusCode == http.StatusNotModified {
+		return list, BuildResponse(r), nil
+	}
+	if jsonErr := json.NewDecoder(r.Body).Decode(&list); jsonErr != nil {
+		return nil, nil, NewAppError("GetFileInfosForPostIncludeDeleted", "api.unmarshal_error", nil, jsonErr.Error(), http.StatusInternalServerError)
+	}
+	return list, BuildResponse(r), nil
+}
+
 // General/System Section
 
 // GenerateSupportPacket downloads the generated support packet