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

utils/utils.go

@@ -7,11 +7,9 @@ import (
 	"net"
 	"net/http"
 	"net/url"
-	"os"
 	"strings"
 
 	"github.com/mattermost/mattermost-server/model"
-	"github.com/mattermost/mattermost-server/utils/fileutils"
 )
 
 func StringInSlice(a string, slice []string) bool {
@@ -40,17 +38,6 @@ func StringArrayIntersection(arr1, arr2 []string) []string {
 	return result
 }
 
-func FileExistsInConfigFolder(filename string) bool {
-	if len(filename) == 0 {
-		return false
-	}
-
-	if _, err := os.Stat(fileutils.FindConfigFile(filename)); err == nil {
-		return true
-	}
-	return false
-}
-
 func RemoveDuplicatesFromStringArray(arr []string) []string {
 	result := make([]string, 0, len(arr))
 	seen := make(map[string]bool)