[MM-17069] Api endpoint to unread a channel (#11794)

* [MM-17069] endpoint to unread a channel from post

* [MM-17069] update mock

* [MM-17069] first passing test

* [MM-17069] fix SQL typo

* [MM-17069] fix msgCount

* add tests

* [MM-17069] Fix tests

* [MM-16069] Remove trash, add comments

* [MM-16069] Add message to errors

* [MM-17069] fix go fmt

* [MM-17069] return an UnreadChannel response

* [MM-17069] added unauthorized test

* [MM-17069] fix operator

* [MM-17069] refactor tests

* [MM-16069] back to green tests

* [MM-17069] change url to include user

* [MM-17069] Fixing code review comments

* [MM-17069] One shouldn't fix manually what a machine can fix better

* [MM-17069] change response type, update tests

* [MM-17069] fix permission error

* [MM-17069] Add tests for edit_other_users permission

* [MM-17069] no magic numbers

api4/post.go

@@ -28,6 +28,7 @@ func (api *API) InitPost() {
 	api.BaseRoutes.Team.Handle("/posts/search", api.ApiSessionRequired(searchPosts)).Methods("POST")
 	api.BaseRoutes.Post.Handle("", api.ApiSessionRequired(updatePost)).Methods("PUT")
 	api.BaseRoutes.Post.Handle("/patch", api.ApiSessionRequired(patchPost)).Methods("PUT")
+	api.BaseRoutes.PostForUser.Handle("/set_unread", api.ApiSessionRequired(setPostUnread)).Methods("POST")
 	api.BaseRoutes.Post.Handle("/pin", api.ApiSessionRequired(pinPost)).Methods("POST")
 	api.BaseRoutes.Post.Handle("/unpin", api.ApiSessionRequired(unpinPost)).Methods("POST")
 }
@@ -576,6 +577,28 @@ func patchPost(c *Context, w http.ResponseWriter, r *http.Request) {
 	w.Write([]byte(patchedPost.ToJson()))
 }
 
+func setPostUnread(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequirePostId().RequireUserId()
+	if c.Err != nil {
+		return
+	}
+	if c.App.Session.UserId != c.Params.UserId && !c.App.SessionHasPermissionToUser(c.App.Session, c.Params.UserId) {
+		c.SetPermissionError(model.PERMISSION_EDIT_OTHER_USERS)
+		return
+	}
+	if !c.App.SessionHasPermissionToChannelByPost(c.App.Session, c.Params.PostId, model.PERMISSION_READ_CHANNEL) {
+		c.SetPermissionError(model.PERMISSION_READ_CHANNEL)
+		return
+	}
+
+	state, err := c.App.MarkChannelAsUnreadFromPost(c.Params.PostId, c.Params.UserId)
+	if err != nil {
+		c.Err = err
+		return
+	}
+	w.Write([]byte(state.ToJson()))
+}
+
 func saveIsPinnedPost(c *Context, w http.ResponseWriter, r *http.Request, isPinned bool) {
 	c.RequirePostId()
 	if c.Err != nil {