Have both `C-c C-'` and `C-c '` bound to `helm-do-ag-project-root`, so
it can be triggered from a terminal. As `C-'` cannot be sent to emacs
due to limitations of terminal emulators.
Also move `helm-do-ag` from `C-c C-s` to `<helm-prefix> a`, now that the
`helm-command-map` can reliably be used. This effectively means you
trigger `helm-do-ag` with `C-c h a` right now.