MM-27493 Shared channels (MVP) (#17301)

Remote Cluster Service
- provides ability for multiple Mattermost cluster instances to create a trusted connection with each other and exchange messages
- trusted connections are managed via slash commands (for now)
- facilitates features requiring inter-cluster communication, such as Shared Channels
Shared Channels Service
- provides ability to shared channels between one or more Mattermost cluster instances (using trusted connection)
- sharing/unsharing of channels is managed via slash commands (for now)

model/utils_test.go

@@ -40,6 +40,14 @@ func TestGetMillisForTime(t *testing.T) {
 	require.Equalf(t, thisTimeMillis, result, "millis are not the same: %d and %d", thisTimeMillis, result)
 }
 
+func TestGetTimeForMillis(t *testing.T) {
+	thisTimeMillis := int64(1471219200000)
+	thisTime := time.Date(2016, time.August, 15, 0, 0, 0, 0, time.UTC)
+
+	result := GetTimeForMillis(thisTimeMillis)
+	require.True(t, thisTime.Equal(result))
+}
+
 func TestPadDateStringZeros(t *testing.T) {
 	for _, testCase := range []struct {
 		Name     string