Add primary_key to issue_index (#16813)

Make the group_id a primary key in issue_index. This already has an unique index
and therefore is a good candidate for becoming a primary key.

This PR also changes all other uses of this table to add the group_id as the
primary key.

Fix #16802

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/migrations/v192.go

@@ -0,0 +1,18 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"xorm.io/xorm"
+)
+
+func recreateIssueResourceIndexTable(x *xorm.Engine) error {
+	type IssueIndex struct {
+		GroupID  int64 `xorm:"pk"`
+		MaxIndex int64 `xorm:"index"`
+	}
+
+	return RecreateTables(new(IssueIndex))(x)
+}