Inside a subgroup scope, when the user clicks on the
search drop down in the header, some options are displayed.

Some of this links direct the user to the issues and merge
requests linked to the subgroup. Nevertheless, the url
was broken because they doesn't include the complete
group namespace

The "basename" here needs to be the full path without the trailing
extension, instead of stripping the leading path as well.

This was previously fixed in 2f36efa0871 inside the view, but the
problematic code was still present in FoundBlob, and the corresponding
spec didn't actually use a child wiki page to properly verify the fix.

Fix whitespace in specs because normalize_ws is slightly
different from Capybara 2 behavior

This commit contains the patch suggested by @fjsanpedro in the issue
covering broken links in wiki search results.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/56234

Backports https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10161
(code out of ee/ folder).

Extract language_from_git_attributes as a concern so it can
ben included in two blob classes.