frontend, api: allow custom emails as commenters

I'm really sorry this came out as one huge commit, but I'm afraid
I can't split this up.

Closes https://gitlab.com/commento/commento-ce/issues/9

api/commenter_login_test.go

@@ -0,0 +1,58 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestCommenterLoginBasics(t *testing.T) {
+	failTestOnError(t, setupTestEnv())
+
+	if _, err := commenterLogin("test@example.com", "hunter2"); err == nil {
+		t.Errorf("expected error not found when logging in without creating an account")
+		return
+	}
+
+	commenterNew("test@example.com", "Test", "undefined", "undefined", "commento", "hunter2")
+
+	if _, err := commenterLogin("test@example.com", "hunter2"); err != nil {
+		t.Errorf("unexpected error when logging in: %v", err)
+		return
+	}
+
+	if _, err := commenterLogin("test@example.com", "h******"); err == nil {
+		t.Errorf("expected error not found when given wrong password")
+		return
+	}
+
+	if session, err := commenterLogin("test@example.com", "hunter2"); session == "" {
+		t.Errorf("empty session on successful login: %v", err)
+		return
+	}
+}
+
+func TestCommenterLoginEmpty(t *testing.T) {
+	failTestOnError(t, setupTestEnv())
+
+	if _, err := commenterLogin("test@example.com", ""); err == nil {
+		t.Errorf("expected error not found when passing empty password")
+		return
+	}
+
+	commenterNew("test@example.com", "Test", "undefined", "", "commenter", "hunter2")
+
+	if _, err := commenterLogin("test@example.com", ""); err == nil {
+		t.Errorf("expected error not found when passing empty password")
+		return
+	}
+}
+
+func TestCommenterLoginNonCommento(t *testing.T) {
+	failTestOnError(t, setupTestEnv())
+
+	commenterNew("test@example.com", "Test", "undefined", "undefined", "google", "")
+
+	if _, err := commenterLogin("test@example.com", "hunter2"); err == nil {
+		t.Errorf("expected error not found logging into a non-Commento account")
+		return
+	}
+}