Consider allowing ES to not index code

We want to enable Elasticsearch on GitLab.com: &153 (closed)

Our index is too big, due to the code search: &429 (closed)

One option to get something enabled on GitLab.com earlier would be to just allow a configuration option to not include code search. This wouldn't be super-useful from a user perspective (although still better than SQL search performance and wise), but it's a nice half-way step.