[6.0] Refactor UNION queries

[staudenmeir]

SQL Server is the only database left where we don't wrap the subqueries of UNION queries in parentheses. We can make it the default behavior and remove a lot of duplicate code.

This allows more complex queries on SQL Server:

DB::table('posts')->latest()->limit(5)->unionAll(
    DB::table('videos')->latest()->limit(5)
)->latest()->get();

select * from (
  select top 5 * from [posts] order by [created_at] desc
) as [temp_table]
union all
select * from (
  select top 5 * from [videos] order by [created_at] desc
) as [temp_table]
order by [created_at] desc

The new wrapUnion() method handles the different ways we wrap the subqueries:
(...) vs. select * from (...)