frontend, api, db: add single sign-on

Closes https://gitlab.com/commento/commento/issues/90

db/20190420181913-sso.sql

@@ -0,0 +1,10 @@
+-- Single Sign-On (SSO)
+
+ALTER TABLE domains
+  ADD ssoProvider BOOLEAN NOT NULL DEFAULT false;
+
+ALTER TABLE domains
+  ADD ssoSecret TEXT NOT NULL DEFAULT '';
+
+ALTER TABLE domains
+  ADD ssoUrl TEXT NOT NULL DEFAULT '';