Move all mail related codes from models to services/mailer (#7200)

* move all mail related codes from models to modules/mailer

* fix lint

* use DBContext instead Engine

* use WithContext not WithEngine

* Use DBContext instead of Engine

* don't use defer when sess.Close()

* move DBContext to context.go and add some methods

* move mailer from modules/ to services

* fix lint

* fix tests

* fix fmt

* add gitea copyright

* fix tests

* don't expose db functions

* make code clear

* add DefaultDBContext

* fix build

* fix bug

models/context.go

@@ -0,0 +1,55 @@
+// Copyright 2019 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 models
+
+// DBContext represents a db context
+type DBContext struct {
+	e Engine
+}
+
+// DefaultDBContext represents a DBContext with default Engine
+func DefaultDBContext() DBContext {
+	return DBContext{x}
+}
+
+// Committer represents an interface to Commit or Close the dbcontext
+type Committer interface {
+	Commit() error
+	Close()
+}
+
+// TxDBContext represents a transaction DBContext
+func TxDBContext() (DBContext, Committer, error) {
+	sess := x.NewSession()
+	if err := sess.Begin(); err != nil {
+		sess.Close()
+		return DBContext{}, nil, err
+	}
+
+	return DBContext{sess}, sess, nil
+}
+
+// WithContext represents executing database operations
+func WithContext(f func(ctx DBContext) error) error {
+	return f(DBContext{x})
+}
+
+// WithTx represents executing database operations on a trasaction
+func WithTx(f func(ctx DBContext) error) error {
+	sess := x.NewSession()
+	if err := sess.Begin(); err != nil {
+		sess.Close()
+		return err
+	}
+
+	if err := f(DBContext{sess}); err != nil {
+		sess.Close()
+		return err
+	}
+
+	err := sess.Commit()
+	sess.Close()
+	return err
+}