Add more database indexes

[Toflar]

Fixes #6681

@contao/developers I've only added indexes to the columns that were part of my slow query log. Of course, that will probably be different in other setups. An alternative would be to automate adding the indexes for columns such as

• id
• pid
• sorting
• ptable
• fields with relation of 'type'=>'hasOne' or 'type'=>'belongsTo'

Not sure if we'd end up having too many indexes?

[ausi]

I would not add the index tl_content.ptable and tl_page.dns,fallback,published,start,stop, see code comments.

The rest LGTM 🚀

[ausi]

Approved, apart from #6683 (comment)

[leofeyer]

Thank you @Toflar.