Timezone feature (#8185)

* Add supported timezones into config

Remove Timezone list creation in favor of timezone from configs

Add Timezone field to Users table

Clean up format of SupportedTimezones in config

* Remove unwanted change

* Add test for updating user timezone

* Add empty map[string]string if Timezone is null

* Add EnableTimezoneSelection config

* Revert back to map[string]string for ClientConfig

* Refactor SupportedTimezones into timezones.json

* Include timezones.json in TestConfigFlag

* Add timezone api endpoint

* Bump varchar size to 256 and setMaxSize in user_store

* Refactor LoadConfig to LoadConfig and LoadTimezoneConfig

* Remove unnecessary argument in LoadConfig, mail_test

* Add test for timezone endpoint

* Add license header

* Refactor timezones endpoint to system.go

* Add system base route to timezone endpoint

* db timezone upgrade in db v4.9

* Avoid saving SupportedTimezones to config.json

* Add timezonePath support in config

* Remove EnableTimezoneSelection from config

* Use return statement without return parameter

* Refactor test for SupportedTimezones

* Check for supportedTimezone != nil instead of using len

* Decouple SupportedTimezones out of Config

* Fix failing test

* Add LastTeamIconUpdate back in upgrade.go

* Write timezone config in config_flag_test

* Add code fallback for default timezone support

cmd/commands/config_flag_test.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/stretchr/testify/require"
 
+	"encoding/json"
 	"github.com/mattermost/mattermost-server/cmd"
 	"github.com/mattermost/mattermost-server/utils"
 )
@@ -26,6 +27,11 @@ func TestConfigFlag(t *testing.T) {
 	configPath := filepath.Join(dir, "foo.json")
 	require.NoError(t, ioutil.WriteFile(configPath, []byte(config.ToJson()), 0600))
 
+	timezones := utils.LoadTimezones("timezones.json")
+	tzConfigPath := filepath.Join(dir, "timezones.json")
+	timezoneData, _ := json.Marshal(timezones)
+	require.NoError(t, ioutil.WriteFile(tzConfigPath, timezoneData, 0600))
+
 	i18n, ok := utils.FindDir("i18n")
 	require.True(t, ok)
 	require.NoError(t, utils.CopyDir(i18n, filepath.Join(dir, "i18n")))