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

config/file_test.go

@@ -615,26 +615,13 @@ func TestFileStoreSave(t *testing.T) {
 		},
 	}
 
-	t.Run("set without save", func(t *testing.T) {
+	t.Run("set with automatic save", func(t *testing.T) {
 		_, err = fs.Set(newCfg)
 		require.NoError(t, err)
 
 		err = fs.Load()
 		require.NoError(t, err)
 
-		assert.Equal(t, "http://minimal", *fs.Get().ServiceSettings.SiteURL)
-	})
-
-	t.Run("set with save", func(t *testing.T) {
-		_, err = fs.Set(newCfg)
-		require.NoError(t, err)
-
-		err = fs.Save()
-		require.NoError(t, err)
-
-		err = fs.Load()
-		require.NoError(t, err)
-
 		assert.Equal(t, "http://new", *fs.Get().ServiceSettings.SiteURL)
 	})
 }