MM-39710: saved posts screen and DB (#6020)

* MM-39710: saved posts screen and DB

- Adds ids of saved posts to the systems table, as we do with recent
mentions.
- Adds a new remote action to fetch saved posts (getFlaggedPosts).
- Adds a new screen to display those in a mobile.
- Displays saved posts in the tablet view next to profile card.

* Uses Preferences instead of System table

Renames to saved posts wherever possible

* Adds text to localization file

* Fixes fetching/saving saved posts

* Refactor mini post to components folder

* Fixes hooks dependencies according to review

* Removes unnecessary 'withObservables'

* Small refactor

* Satisfies linter

* Adds empty state

And fixes empty state icon to be theme sensitive.
Both recent_mentions and saved_posts.

* Fixes empty screen's alignment

* Add missing preference

* add missing translation strings

* remove unused database type definition

* Fetch newly saved post

* Fix return type for client.getSavedPosts

* Remove usage of lodash compose

* Rename get remote actions to fetch

* Include close button for savedPost modal

* fix tablet view for SavePosts and use lottie loading indicator

* Render post with content for save posts and recent mentions

* post list viewable items type definition

* Add layout width to post content for saved post screen

* Use PostWithChannel and viewableItems for saved posts and recent mentions

* Layout margin of 20

* openGraphImage margin

* Fix openGraphImage display

Co-authored-by: Elias Nahum <nahumhbl@gmail.com>

types/components/post_list.d.ts

@@ -0,0 +1,11 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+import {ViewToken} from 'react-native';
+
+export type ViewableItemsChanged = {
+    viewableItems: ViewToken[];
+    changed: ViewToken[];
+}
+
+export type ViewableItemsChangedListenerEvent = (viewableItms: ViewToken[]) => void;