Feature/search after before on (#9219)

* initial implementation of after, before, on search flags allowing to restrict the search to a specific day or a date range

* missed setting beforeDate in SearchParams in one place

* fixed condition when only flags are used for search without any plain terms

* changed date format used for after/before/on flags to be in ISO8601 format as suggested in PR comments, added a helper function to pad month and day with zeroes allowing the user user either format, with or without leading zeroes

* corrected expected compare to date setting for the TestParseDateFilterToTimeISO8601 test

* fixed a bug for the scenario when you only have the date flags without any terms, added a couple of tests for that scenario

* updated the date filter logic to use parameters to construct the query instead of simply appending strings together, as suggested in the pull request comments

* added search unit test using date flags

* added a helper function to create a test post with a createat date manually set, updated the test for search using date flags to create test posts with different createat dates to be able to better test the functionality

* MM-11817 Add support for after/before/on search flags with Elasticsearch

* add support to search posts to perform the search in context of the client's timezone when filtering by createat date using on: after: before: flags

* updated tests to match the new signature

app/post.go

@@ -646,8 +646,8 @@ func (a *App) DeletePostFiles(post *model.Post) {
 	}
 }
 
-func (a *App) SearchPostsInTeam(terms string, userId string, teamId string, isOrSearch bool, includeDeletedChannels bool) (*model.PostSearchResults, *model.AppError) {
-	paramsList := model.ParseSearchParams(terms)
+func (a *App) SearchPostsInTeam(terms string, userId string, teamId string, isOrSearch bool, includeDeletedChannels bool, timeZoneOffset int) (*model.PostSearchResults, *model.AppError) {
+	paramsList := model.ParseSearchParams(terms, timeZoneOffset)
 	includeDeleted := includeDeletedChannels && *a.Config().TeamSettings.ExperimentalViewArchivedChannels
 
 	esInterface := a.Elasticsearch