Elasticsearch filter by programming language

Like https://gitlab.com/gitlab-org/gitlab-ee/issues/5024#note_60304551 and https://gitlab.com/gitlab-org/gitlab-ee/issues/1052

Currently, we put lots of work into analyzing indexed files and storing a guess at the language they contain ("Ruby", "Go", "JavaScript", etc), but we don't expose this field anywhere, so it's wasted effort and index space.

Rather than removing it, let's add a filter like language:ruby. It would greatly increase the value added to search by elasticsearch integration, since this isn't something we can reasonably do without it, even when searching a single project.

Github's syntax: https://help.github.com/articles/searching-code/#search-by-language

/cc @smcgivern @victorwu