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/index.go

@@ -14,7 +14,7 @@ import (
 // ResourceIndex represents a resource index which could be used as issue/release and others
 // We can create different tables i.e. issue_index, release_index and etc.
 type ResourceIndex struct {
-	GroupID  int64 `xorm:"unique"`
+	GroupID  int64 `xorm:"pk"`
 	MaxIndex int64 `xorm:"index"`
 }