Allow user-select in ResultContainer header

Created by: pariser

Often my goal in doing a search in Sourcegraph is to find files where I need to make some kind of changes in my editor. To that end, my lack of ability to copy filenames from headers is super frustrating, so much so that I came here to submit this feature request. I know I'm not the only one of my coworkers who has been frustrated by this.

I hope this isn't a controversial change but understand that there may be further implications of this change that might not be ideal. Please let me know if there are further things to do to help move this along.

Test plan: Trivial change, but I'd be happy to follow instructions to verify this somehow.