[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>

api4/post.go

@@ -898,7 +898,13 @@ func getFileInfosForPost(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	infos, err := c.App.GetFileInfosForPostWithMigration(c.Params.PostId)
+	includeDeleted, _ := strconv.ParseBool(r.URL.Query().Get("include_deleted"))
+	if includeDeleted && !c.App.SessionHasPermissionTo(*c.AppContext.Session(), model.PermissionManageSystem) {
+		c.SetPermissionError(model.PermissionManageSystem)
+		return
+	}
+
+	infos, err := c.App.GetFileInfosForPostWithMigration(c.Params.PostId, includeDeleted)
 	if err != nil {
 		c.Err = err
 		return