[MM-32390] Config logic refactor (#17578)

* Replace config generator

* Cleanup

* Some renaming and docs additions to add clarity

* Cleanup logging related methods

* Cleanup emitter

* Fix TestDefaultsGenerator

* Move feature flags synchronization logic out of config package

* Remove unnecessary util functions

* Simplify load/set logic

* Refine semantics and add some test to cover them

* Remove unnecessary deep copies

* Improve logic further

* Fix license header

* Review file store tests

* Fix test

* Fix test

* Avoid additional write during initialization

* More consistent naming

* Update app/feature_flags.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

* Move FF synchronizer to its own package

* Remove unidiomatic use of sync.Once

* Add some comments

* Rename function

* More comment

Co-authored-by: Christopher Speller <crspeller@gmail.com>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

scripts/config_generator/main.go

@@ -0,0 +1,46 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"github.com/mattermost/mattermost-server/v5/model"
+)
+
+// generateDefaultConfig writes default config to outputFile.
+func generateDefaultConfig(outputFile *os.File) error {
+	defaultCfg := &model.Config{}
+	defaultCfg.SetDefaults()
+	if data, err := json.MarshalIndent(defaultCfg, "", "  "); err != nil {
+		return err
+	} else if _, err := outputFile.Write(data); err != nil {
+		return err
+	}
+	return nil
+}
+
+func main() {
+	outputFile := os.Getenv("OUTPUT_CONFIG")
+	if outputFile == "" {
+		fmt.Println("Output file name is missing. Please set OUTPUT_CONFIG env variable to absolute path")
+		os.Exit(2)
+	}
+	if _, err := os.Stat(outputFile); !os.IsNotExist(err) {
+		_, _ = fmt.Fprintf(os.Stderr, "File %s already exists. Not overwriting!\n", outputFile)
+		os.Exit(2)
+	}
+
+	if file, err := os.Create(outputFile); err == nil {
+		err = generateDefaultConfig(file)
+		_ = file.Close()
+		if err != nil {
+			panic(err)
+		}
+	} else {
+		panic(err)
+	}
+}