search: create global text search job

Created by: rvantonder

This creates a global text search job. It's a big (and happy) shift. After this, only the global symbols text job remains to be done.

This change is net additions, but it's only because I'm holding off on deleting a ton of unused stuff once the symbol search job exists too.

Tests look good (I did push a backend-integration build job after misnaming this branch :P). I'll be monitoring search blitz and such for stability/correctness on Sourcegraph.com after merging just in case there is some oversight that affects perf.