Disable script access to `_source` by default

[clintongormley]

Accessing the _source field within a script is very slow and can hurt a cluster's performance. That said, it can be a useful way of accessing some not-indexed value for one-off uses without having to reindex.

As a safeguard, let's disable script access to _source by default (other than update/reindex scripts, obviously) and allow the user to re-enable access with a dynamic index setting.

[nik9000]

I was against this when I read the title but I'm ok with it if we make it enable-able with a dynamic setting. _source is genuinely useful but really slow. It'd be nice to document exactly why when we write the docs for the dynamic setting. Maybe this is a thing we should enable on the request instead? It feels like a dynamic setting is too broad. Like, when I ran _source things at WMF I really only ran them on a handful of requests. All hand written, all ones I was willing to wait for. OTOH allowing anyone to override it feels funky. If core ES had users and permissions I'd say "this should be a permission a user has".

Point of clarification: this only applies to searches. It'll be ok by default to put it in _update and the mutation script in _update_by_query or _reindex.

[clintongormley]

Maybe this is a thing we should enable on the request instead? It feels like a dynamic setting is too broad.

Think of this from the viewpoint of the sysadmin who wants to protect their cluster from abuse by users. Like the other safeguards, this would be a setting the sysadmin could use to turn slow/dangerous stuff off