Sync gitea app path for git hooks and authorized keys when starting (#17335)

Gitea writes its own AppPath into git hook scripts. If Gitea's AppPath changes, then the git push will fail.

This PR:

* Introduce an AppState module, it can persist app states into database
* During GlobalInit, Gitea will check if the current AppPath is the same as last one. If they don't match, Gitea will sync git hooks.
* Refactor some code to make them more clear.
* Also, "Detect if gitea binary's name changed" #11341 is related, we call models.RewriteAllPublicKeys to update ssh authorized_keys file

modules/appstate/db.go

@@ -0,0 +1,37 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package appstate
+
+import (
+	"code.gitea.io/gitea/models/appstate"
+	"code.gitea.io/gitea/modules/json"
+
+	"github.com/yuin/goldmark/util"
+)
+
+// DBStore can be used to store app state items in local filesystem
+type DBStore struct {
+}
+
+// Get reads the state item
+func (f *DBStore) Get(item StateItem) error {
+	content, err := appstate.GetAppStateContent(item.Name())
+	if err != nil {
+		return err
+	}
+	if content == "" {
+		return nil
+	}
+	return json.Unmarshal(util.StringToReadOnlyBytes(content), item)
+}
+
+// Set saves the state item
+func (f *DBStore) Set(item StateItem) error {
+	b, err := json.Marshal(item)
+	if err != nil {
+		return err
+	}
+	return appstate.SaveAppStateContent(item.Name(), util.BytesToReadOnlyString(b))
+}