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/v182_test.go

@@ -33,8 +33,8 @@ func Test_addIssueResourceIndexTable(t *testing.T) {
 	}
 
 	type ResourceIndex struct {
-		GroupID  int64 `xorm:"index unique(s)"`
-		MaxIndex int64 `xorm:"index unique(s)"`
+		GroupID  int64 `xorm:"pk"`
+		MaxIndex int64 `xorm:"index"`
 	}
 
 	var start = 0