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).
API Make HTMLValue replace-able via DI

Extracted common code out to SS_HTMLValue and made abstract, then
put HTML 4 specific code in SS_HTML4Value. Its now possible to
replace HTMLValue with one designed for HTML 5 or XHTML

Requires a code change from new SS_HTMLValue to
Injector::inst()->create(HTMLValue)
Make ShortcodeParser use HTMLValue for HTML parsing
FIX Make multiple TemplateManifests not corrupt each other
NEW Let shortcodes be disabled in HTMLText & HTMLVarchar
NEW Disable shortcodes in $Layout and $Content in SSViewer
API Make default_cast Text and not HTMLText
API Make SSViewer#process return HTMLText not string

This means that you dont have to worry about casting it
as HTMLText again when using the result in a template or other context

However in some situations code might be assuming it can
check with is_string, in which case you now need to use instanceof HTMLText
Update 3.1.0 upgrading notes for casting changes
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.