Filter by filename search with and without Elasticsearch

• Enable filter by filename in search, using a syntax similar to that of GitHub: https://help.github.com/articles/searching-code/#search-by-filename
• If the instance has Elasticsearch configured on, the search will use Elasticsearch.
  • In this case, the filename search will be in both the project context and the advanced global search context.
• If the instance does not have Elasticsearch configured on, the search will not use it.
  • In this case, the filename search will only work where code search already does - i.e. in the project context only.
• In addition, equivalent functionality should be added to the API.
• Equivalent CE issue is https://gitlab.com/gitlab-org/gitlab-ce/issues/43627

Out of scope

Search by file extension is out of scope. See discussion in comments for context.