New /integrations/sage page

Replacing `/sage` (which now has a redirect - note redirect is to `/partners/sage` which will be final URL - `/integrations` -> `/partners` happening in another PR) - in mean time working address for this PR is still `/integrations/sage`.

Needs:

- Sage logo
- SVG icons

![gocardless for sage improve cashflow with direct debit gocardless](https://cloud.githubusercontent.com/assets/883598/4546367/146610d0-4e43-11e4-9897-571b9922c182.png)

From phil over my shoulder! Prob should just call this "Sage logos" in the alt tag. Or "Sage and windows logos"