Add primary_key to issue_index (#16813) (#16820)

Add primary_key to issue_index (#16813) (#16820)

Backport #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.

The migration v192 from #16813 has not been backported but Xorm will
work fine with non-primary keyed tables. If a user on 1.15 wishes to
have the correct schema sooner than 1.16 - they can use gitea doctor
recreate-table issue_index and gitea will recreate the table with the
primary key.

Fix #16802

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