MM-14143 config cleanup final (#10374)

* TestGetLicenseFileFromDisk: avoid using fileutils.FindConfigFile

* config: abstract config-related file access, extend memory store

* simplify config validate to avoid file knowledge

* fix relative file tests

* cluster: fix ConfigChanged event

The old and new configurations were swapped when notifying the enterprise code of configuration changes, creating needless instability in propagating config updates across a cluster.

* config/database: ignore duplicates

* test cleanup

* remove unnecessary Save() in test

app/config.go

@@ -17,6 +17,8 @@ import (
 	"strconv"
 	"time"
 
+	"github.com/pkg/errors"
+
 	"github.com/mattermost/mattermost-server/config"
 	"github.com/mattermost/mattermost-server/mlog"
 	"github.com/mattermost/mattermost-server/model"
@@ -346,3 +348,13 @@ func (a *App) LimitedClientConfigWithComputed() map[string]string {
 
 	return respCfg
 }
+
+// GetConfigFile proxies access to the given configuration file to the underlying config store.
+func (a *App) GetConfigFile(name string) ([]byte, error) {
+	data, err := a.Srv.configStore.GetFile(name)
+	if err != nil {
+		return nil, errors.Wrapf(err, "failed to get config file %s", name)
+	}
+
+	return data, nil
+}