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

app/config.go

@@ -56,12 +56,6 @@ func (a *App) UpdateConfig(f func(*model.Config)) {
 	a.Srv.UpdateConfig(f)
 }
 
-func (a *App) PersistConfig() {
-	if err := a.Srv.configStore.Save(); err != nil {
-		mlog.Error("Failed to persist config", mlog.Err(err))
-	}
-}
-
 func (s *Server) ReloadConfig() error {
 	debug.FreeOSMemory()
 	if err := s.configStore.Load(); err != nil {