MM-14145: The config store Set will now Save automatically (#10377)

* MM-14145: The config store Set will now Save automatically

When UpdateConfig (and configStore.Set) is called in admin.go and
config.go, commonStore.Set now takes a store-specific persist function.
It uses that persist function to save the configuration automatically.

Removed: Now callers do not have to call configStore.Save or
app.PersistConfig, and those functions have been removed.

Possible downside: this means a "failed to persist config" error can now
be thrown during a app.UpdateConfig or commonStore.Set call. But
considering application code never really sets a config without saving
it (except in the test cases, which were testing that -- see below), it
seems fine to group these responsibilities.

Also removed: tests for 'set without save'. Since that can not happen
anymore, the tests are not needed.

* Removed Save completely, cleaned up formatting, joined save test with
set tests.

* fixed shadowed variable error

cmd/mattermost/commands/config.go

@@ -226,9 +226,6 @@ func configSetCmdF(command *cobra.Command, args []string) error {
 
 	f := updateConfigValue(configSetting, newVal, oldConfig, newConfig)
 	f(newConfig)
-	if _, err := configStore.Set(newConfig); err != nil {
-		return errors.Wrap(err, "failed to set config")
-	}
 
 	// UpdateConfig above would have already fixed these invalid locales, but we check again
 	// in the context of an explicit change to these parameters to avoid saving the fixed
@@ -237,7 +234,9 @@ func configSetCmdF(command *cobra.Command, args []string) error {
 		return errors.New("Invalid locale configuration")
 	}
 
-	configStore.Save()
+	if _, err := configStore.Set(newConfig); err != nil {
+		return errors.Wrap(err, "failed to set config")
+	}
 
 	return nil
 }