feat: implement imprint/impressum support (#706)

* feat: implement imprint/impressum support

Closes #362

Signed-off-by: Xe Iaso <me@xeiaso.net>

* chore(docs/anubis): enable an imprint

Signed-off-by: Xe Iaso <me@xeiaso.net>

* chore: spelling

Signed-off-by: Xe Iaso <me@xeiaso.net>

* docs: fix the end of the sentence, comment out a default impressum

Signed-off-by: Xe Iaso <me@xeiaso.net>

* docs: link back to impressum page

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/policy/config/impressum_test.go

@@ -0,0 +1,62 @@
+package config
+
+import (
+	"bytes"
+	"errors"
+	"testing"
+)
+
+func TestImpressumValid(t *testing.T) {
+	for _, cs := range []struct {
+		name string
+		inp  Impressum
+		err  error
+	}{
+		{
+			name: "basic happy path",
+			inp: Impressum{
+				Footer: "<p>Website hosted by Techaro.<p>",
+				Page: ImpressumPage{
+					Title: "Techaro Imprint",
+					Body:  "<p>This is an imprint page.</p>",
+				},
+			},
+			err: nil,
+		},
+		{
+			name: "no footer",
+			inp: Impressum{
+				Footer: "",
+				Page: ImpressumPage{
+					Title: "Techaro Imprint",
+					Body:  "<p>This is an imprint page.</p>",
+				},
+			},
+			err: ErrMissingValue,
+		},
+		{
+			name: "page not valid",
+			inp: Impressum{
+				Footer: "test page please ignore",
+			},
+			err: ErrMissingValue,
+		},
+	} {
+		t.Run(cs.name, func(t *testing.T) {
+			if err := cs.inp.Valid(); !errors.Is(err, cs.err) {
+				t.Logf("want: %v", cs.err)
+				t.Logf("got:  %v", err)
+				t.Error("validation failed")
+			}
+
+			var buf bytes.Buffer
+			if err := cs.inp.Render(t.Context(), &buf); err != nil {
+				t.Errorf("can't render footer: %v", err)
+			}
+
+			if err := cs.inp.Page.Render(t.Context(), &buf); err != nil {
+				t.Errorf("can't render page: %v", err)
+			}
+		})
+	}
+}