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

api4/apitestlib.go

@@ -444,6 +444,17 @@ func (me *TestHelper) CreateMessagePostWithClient(client *model.Client4, channel
 	return rpost
 }
 
+func (me *TestHelper) CreateMessagePostNoClient(channel *model.Channel, message string, createAtTime int64) *model.Post {
+	post := store.Must(me.App.Srv.Store.Post().Save(&model.Post{
+		UserId:    me.BasicUser.Id,
+		ChannelId: channel.Id,
+		Message:   message,
+		CreateAt:  createAtTime,
+	})).(*model.Post)
+
+	return post
+}
+
 func (me *TestHelper) LoginBasic() {
 	me.LoginBasicWithClient(me.Client)
 }