Implement create and get incoming webhook endpoints for APIv4 (#5407)

* Implement POST /hooks/incoming endpoint for APIv4

* Implement GET /hooks/incoming endpoint for APIv4

* Updates per feedback

store/sql_channel_store.go

@@ -357,7 +357,7 @@ func (s SqlChannelStore) get(id string, master bool, allowFromCache bool) StoreC
 		if obj, err := db.Get(model.Channel{}, id); err != nil {
 			result.Err = model.NewLocAppError("SqlChannelStore.Get", "store.sql_channel.get.find.app_error", nil, "id="+id+", "+err.Error())
 		} else if obj == nil {
-			result.Err = model.NewLocAppError("SqlChannelStore.Get", "store.sql_channel.get.existing.app_error", nil, "id="+id)
+			result.Err = model.NewAppError("SqlChannelStore.Get", "store.sql_channel.get.existing.app_error", nil, "id="+id, http.StatusBadRequest)
 		} else {
 			result.Data = obj.(*model.Channel)
 			channelCache.AddWithExpiresInSecs(id, obj.(*model.Channel), CHANNEL_CACHE_SEC)