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

model/client4.go

@@ -314,6 +314,10 @@ func (c *Client4) GetAnalyticsRoute() string {
 	return fmt.Sprintf("/analytics")
 }
 
+func (c *Client4) GetTimezonesRoute() string {
+	return fmt.Sprintf(c.GetSystemRoute() + "/timezones")
+}
+
 func (c *Client4) DoApiGet(url string, etag string) (*http.Response, *AppError) {
 	return c.DoApiRequest(http.MethodGet, c.ApiUrl+url, "", etag)
 }
@@ -3169,6 +3173,18 @@ func (c *Client4) DeleteReaction(reaction *Reaction) (bool, *Response) {
 	}
 }
 
+// Timezone Section
+
+// GetSupportedTimezone returns a page of supported timezones on the system.
+func (c *Client4) GetSupportedTimezone() (SupportedTimezones, *Response) {
+	if r, err := c.DoApiGet(c.GetTimezonesRoute(), ""); err != nil {
+		return nil, BuildErrorResponse(r, err)
+	} else {
+		defer closeBody(r)
+		return TimezonesFromJson(r.Body), BuildResponse(r)
+	}
+}
+
 // Open Graph Metadata Section
 
 // OpenGraph return the open graph metadata for a particular url if the site have the metadata