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

app/app.go

@@ -63,6 +63,8 @@ type App struct {
 	clientLicenseValue atomic.Value
 	licenseListeners   map[string]func()
 
+	timezones atomic.Value
+
 	siteURL string
 
 	newStore func() store.Store
@@ -125,6 +127,9 @@ func New(options ...Option) (outApp *App, outErr error) {
 		return nil, err
 	}
 	app.EnableConfigWatch()
+
+	app.LoadTimezones()
+
 	if err := utils.InitTranslations(app.Config().LocalizationSettings); err != nil {
 		return nil, errors.Wrapf(err, "unable to load Mattermost translation files")
 	}