[GITEA] rework long-term authentication

- The current architecture is inherently insecure, because you can
construct the 'secret' cookie value with values that are available in
the database. Thus provides zero protection when a database is
dumped/leaked.
- This patch implements a new architecture that's inspired from: [Paragonie Initiative](https://paragonie.com/blog/2015/04/secure-authentication-php-with-long-term-persistence#secure-remember-me-cookies).
- Integration testing is added to ensure the new mechanism works.
- Removes a setting, because it's not used anymore.

(cherry-pick from eff097448b)

Conflicts:

	modules/context/context_cookie.go
	trivial context conflicts

	routers/web/web.go
	ctx.GetSiteCookie(setting.CookieRememberName) moved from services/auth/middleware.go

modules/util/legacy_test.go

@@ -4,8 +4,6 @@
 package util
 
 import (
-	"crypto/aes"
-	"crypto/rand"
 	"fmt"
 	"os"
 	"testing"
@@ -37,21 +35,3 @@ func TestCopyFile(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, testContent, dstContent)
 }
-
-func TestAESGCM(t *testing.T) {
-	t.Parallel()
-
-	key := make([]byte, aes.BlockSize)
-	_, err := rand.Read(key)
-	assert.NoError(t, err)
-
-	plaintext := []byte("this will be encrypted")
-
-	ciphertext, err := AESGCMEncrypt(key, plaintext)
-	assert.NoError(t, err)
-
-	decrypted, err := AESGCMDecrypt(key, ciphertext)
-	assert.NoError(t, err)
-
-	assert.Equal(t, plaintext, decrypted)
-}