Refactor JWT secret generating & decoding code (#29172)

Old code is not consistent for generating & decoding the JWT secrets.

Now, the callers only need to use 2 consistent functions:
NewJwtSecretWithBase64 and DecodeJwtSecretBase64

And remove a non-common function Base64FixedDecode from util.go

modules/generate/generate_test.go

@@ -0,0 +1,34 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package generate
+
+import (
+	"encoding/base64"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDecodeJwtSecretBase64(t *testing.T) {
+	_, err := DecodeJwtSecretBase64("abcd")
+	assert.ErrorContains(t, err, "invalid base64 decoded length")
+	_, err = DecodeJwtSecretBase64(strings.Repeat("a", 64))
+	assert.ErrorContains(t, err, "invalid base64 decoded length")
+
+	str32 := strings.Repeat("x", 32)
+	encoded32 := base64.RawURLEncoding.EncodeToString([]byte(str32))
+	decoded32, err := DecodeJwtSecretBase64(encoded32)
+	assert.NoError(t, err)
+	assert.Equal(t, str32, string(decoded32))
+}
+
+func TestNewJwtSecretWithBase64(t *testing.T) {
+	secret, encoded, err := NewJwtSecretWithBase64()
+	assert.NoError(t, err)
+	assert.Len(t, secret, 32)
+	decoded, err := DecodeJwtSecretBase64(encoded)
+	assert.NoError(t, err)
+	assert.Equal(t, secret, decoded)
+}