add go templating engine for error page and make errors more clear (#260)

ping #199
closes #213

Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/260
Co-authored-by: crapStone <crapstone01@gmail.com>
Co-committed-by: crapStone <crapstone01@gmail.com>

html/html_test.go

@@ -0,0 +1,54 @@
+package html
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSanitizerSimpleString(t *testing.T) {
+	str := "simple text message without any html elements"
+
+	assert.Equal(t, str, sanitizer.Sanitize(str))
+}
+
+func TestSanitizerStringWithCodeTag(t *testing.T) {
+	str := "simple text message with <code>html</code> tag"
+
+	assert.Equal(t, str, sanitizer.Sanitize(str))
+}
+
+func TestSanitizerStringWithCodeTagWithAttribute(t *testing.T) {
+	str := "simple text message with <code id=\"code\">html</code> tag"
+	expected := "simple text message with <code>html</code> tag"
+
+	assert.Equal(t, expected, sanitizer.Sanitize(str))
+}
+
+func TestSanitizerStringWithATag(t *testing.T) {
+	str := "simple text message with <a>a link to another page</a>"
+	expected := "simple text message with a link to another page"
+
+	assert.Equal(t, expected, sanitizer.Sanitize(str))
+}
+
+func TestSanitizerStringWithATagAndHref(t *testing.T) {
+	str := "simple text message with <a href=\"http://evil.site\">a link to another page</a>"
+	expected := "simple text message with a link to another page"
+
+	assert.Equal(t, expected, sanitizer.Sanitize(str))
+}
+
+func TestSanitizerStringWithImgTag(t *testing.T) {
+	str := "simple text message with a <img alt=\"not found\" src=\"http://evil.site\">"
+	expected := "simple text message with a "
+
+	assert.Equal(t, expected, sanitizer.Sanitize(str))
+}
+
+func TestSanitizerStringWithImgTagAndOnerrorAttribute(t *testing.T) {
+	str := "simple text message with a <img alt=\"not found\" src=\"http://evil.site\" onerror=\"alert(secret)\">"
+	expected := "simple text message with a "
+
+	assert.Equal(t, expected, sanitizer.Sanitize(str))
+}