MM-11210 Add "GET /posts/unread" API to support landing on the last unread post (#11486)

* [MM-11210] Add API GET 'api/v4/channels/{channel_id:[A-Za-z0-9]+}/posts/unread' for scrolling overhaul (#9108)

* Add API GET 'api/v4/channels/{channel_id:[A-Za-z0-9]+}/posts/unread'

* add constants

* refactor GetPostSince and added more tests

* move constants to app package

* [MM-11528 &&  MM-11583] Add userId to in the "posts/unread" path and update test with time delay to fix intermittent failure (#9229)

* add userId to in the "posts/unread" path and update test with time delay to fix intermittent failure

* add limit before and after to query

* remove time delay on test and put pretermined value of Post.CreateAt

* Fix conflict

* [MM-11876] Add cursor to posts list such as next_post_id and previous_post_id (#9707)

* add cursor to posts list such as next_post_id and previous_post_id

add publish previous_post_id on WEBSOCKET_EVENT_POSTED and only get next or previous post IDs if necessary

revert change on adding previous_post_id in WEBSOCKET_EVENT_POSTED

add missing strings import

fix merge conflicts

* update per comment

* update per feedback

* corrected the logic in getting the next and previous post ID

* fix logic to determine next and post IDs, and rename function to have suffix of "Time"

* rearrange logics and add mote tests

* fix merge conflict

* fix missing message when using unread API (#10233)

* MM-15569 Fixes failing test on TestGetPostsForChannelAroundLastUnread (#11039)

* Fix missing posts when getting posts since

* revert changes to GetPostsSince

* migrate Post.GetPostAfterTime and Post.GetPostBeforeTime to sync by default

* revert change to cacheItem

* Fix post ID validation, build query on squirrel and only return post ID as necessary

app/post_metadata.go

@@ -40,8 +40,10 @@ func (a *App) InitPostMetadata() {
 
 func (a *App) PreparePostListForClient(originalList *model.PostList) *model.PostList {
 	list := &model.PostList{
-		Posts: make(map[string]*model.Post, len(originalList.Posts)),
-		Order: originalList.Order, // Note that this uses the original Order array, so it isn't a deep copy
+		Posts:      make(map[string]*model.Post, len(originalList.Posts)),
+		Order:      originalList.Order,
+		NextPostId: originalList.NextPostId,
+		PrevPostId: originalList.PrevPostId,
 	}
 
 	for id, originalPost := range originalList.Posts {