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/permalink.go

@@ -0,0 +1,34 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import "encoding/json"
+
+type Permalink struct {
+	PreviewPost *PreviewPost `json:"preview_post"`
+}
+
+type PreviewPost struct {
+	PostID             string `json:"post_id"`
+	Post               *Post  `json:"post"`
+	TeamName           string `json:"team_name"`
+	ChannelDisplayName string `json:"channel_display_name"`
+}
+
+func NewPreviewPost(post *Post, team *Team, channel *Channel) *PreviewPost {
+	if post == nil {
+		return nil
+	}
+	return &PreviewPost{
+		PostID:             post.Id,
+		Post:               post,
+		TeamName:           team.Name,
+		ChannelDisplayName: channel.DisplayName,
+	}
+}
+
+func (o *Permalink) ToJson() string {
+	b, _ := json.Marshal(o)
+	return string(b)
+}