Don't use Zend_Cache in manifests

The overhead of Zend_Cache in manifests is too high - we don't
need LRU or tags, just somewhere to dump a bunch of data that
persists

You can replace the class used by defining SS_MANIFESTCACHE
to be a class that implements the ManifestCache interface
(we can't use the Config system to set this, as it isn't
initialised yet).
CMS CSS Minor Polishing Tweaks

Fixed what I believe to be a few very minor CSS regressions, that appeared after the CSS restructure for the side-by-side preview.

- Reverted background of the right panel (and tab active state) to the slightly darker shade (as per 3.0) to keep each of the 3 panels visually separate.

- Slightly increased padding on ui-tabs-panel as felt a but too close for comfort. Had decreased since 3.0.

- Decreased padding for logged in user name in menu, felt too excessive. (3.0 was neater)

- Evened out padding above buttons in site tree sidebar

Screenshots showing changes:
3.0: http://spdr.me/xauh
3.1 before commit: http://spdr.me/jkIe
3.1 after commit: http://spdr.me/IxtB
Merge pull request #1287 from silverstripe-rebelalliance/feature/config

Don't use Zend_Cache in manifests
Require config manifests in Core.php to avoid upgrading issues

They need to be present for the autoloader to rebuild
the manifest in the first place.