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

model/utils_test.go

@@ -4,9 +4,11 @@
 package model
 
 import (
+	"fmt"
 	"net/http"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -30,6 +32,39 @@ func TestRandomString(t *testing.T) {
 	}
 }
 
+func TestGetMillisForTime(t *testing.T) {
+	thisTimeMillis := int64(1471219200000)
+	thisTime := time.Date(2016, time.August, 15, 0, 0, 0, 0, time.UTC)
+
+	result := GetMillisForTime(thisTime)
+
+	if thisTimeMillis != result {
+		t.Fatalf(fmt.Sprintf("millis are not the same: %d and %d", thisTimeMillis, result))
+	}
+}
+
+func TestParseDateFilterToTimeISO8601(t *testing.T) {
+	testString := "2016-08-01"
+	compareTime := time.Date(2016, time.August, 1, 0, 0, 0, 0, time.UTC)
+
+	result := ParseDateFilterToTime(testString)
+
+	if result != compareTime {
+		t.Fatalf(fmt.Sprintf("parsed date doesn't match the expected result: parsed result %v and expected time %v", result, compareTime))
+	}
+}
+
+func TestParseDateFilterToTimeNeedZeroPadding(t *testing.T) {
+	testString := "2016-8-1"
+	compareTime := time.Date(2016, time.August, 1, 0, 0, 0, 0, time.UTC)
+
+	result := ParseDateFilterToTime(testString)
+
+	if result != compareTime {
+		t.Fatalf(fmt.Sprintf("parsed date doesn't match the expected result: parsed result %v and expected time %v", result, compareTime))
+	}
+}
+
 func TestAppError(t *testing.T) {
 	err := NewAppError("TestAppError", "message", nil, "", http.StatusInternalServerError)
 	json := err.ToJson()