pgsql schema: Introduce id' and group_id'

Required to ensure referential integrity when renaming groups.

refs #8826

etc/schema/pgsql.schema.sql

@@ -1,6 +1,7 @@
 /* Icinga Web 2 | (c) 2013-2015 Icinga Development Team | GPLv2+ */
 
 CREATE TABLE "icingaweb_group" (
+  "id"     serial,
   "name"   character varying(64) NOT NULL,
   "parent" character varying(64) NULL DEFAULT NULL,
   "ctime"  timestamp NULL DEFAULT NULL,
@@ -10,7 +11,7 @@ CREATE TABLE "icingaweb_group" (
 ALTER TABLE ONLY "icingaweb_group"
   ADD CONSTRAINT pk_icingaweb_group
   PRIMARY KEY (
-    "name"
+    "id"
 );
 
 CREATE UNIQUE INDEX idx_icingaweb_group
@@ -20,23 +21,25 @@ CREATE UNIQUE INDEX idx_icingaweb_group
 );
 
 CREATE TABLE "icingaweb_group_membership" (
-  "group_name" character varying(64) NOT NULL,
+  "group_id"   int NOT NULL,
   "username"   character varying(64) NOT NULL,
   "ctime"      timestamp NULL DEFAULT NULL,
   "mtime"      timestamp NULL DEFAULT NULL
 );
 
 ALTER TABLE ONLY "icingaweb_group_membership"
   ADD CONSTRAINT pk_icingaweb_group_membership
-  PRIMARY KEY (
-    "group_name",
-    "username"
+  FOREIGN KEY (
+    "group_id"
+  )
+  REFERENCES "icingaweb_group" (
+    "id"
 );
 
 CREATE UNIQUE INDEX idx_icingaweb_group_membership
   ON "icingaweb_group_membership"
   USING btree (
-    lower((group_name)::text),
+    group_id,
     lower((username)::text)
 );