Use Search::GlobalService.new in the `GET /projects/search/:query` endpoint

Also streamline the sorting part while we're at it.

That being done, there's currently a duplication between
`GET /projects/search/:query` and `GET /projects?search=:search`
so we might want to keep only the latter for 9.0...

Signed-off-by: Rémy Coutable <remy@rymai.me>