File search improvements

A couple file search improvements requested by a customer:

- [x] Multi-token support (eg, allow searching for both "controller" and "banner" with space in the middle)
- [ ] Regexp support

Start by locating the file-search entry point and reading how the completed multi-token behavior is implemented. The issue names no file or test, so repository search is needed first; done means regexp searches work alongside the existing file-search behavior and are covered by the relevant checks.

索引モデルが issue の本文から書いたものです。