elasticsearch: Cannot index large snippets

Observed while working on https://gitlab.com/gitlab-org/gitlab-ee/issues/11419

To bulk-import snippets, we use rake gitlab:elastic:index_snippets and the es_import helper. This reliably times out on staging.gitlab.com

It looks like the problem occurs when a batch contains a large-enough snippet. At least one in the database is ~5MiB in size.

I'd suggest we exclude or truncate the content for larger snippets. More generally, do we have size limits on snippets? Should we?