Oembed warning if GD function is missing

Silencing errors from a function is terrible so I've stopped that.

I've also added detection for if `imagecreatefromstring` is a declared function and a useful error is thrown if not
FIX: corrected link to CMS Alternating Button Page
Merge pull request #3082 from PapaBearNZ/patch-1

FIX: corrected link to CMS Alternating Button Page
FIX: parent::init is not called first

In the controller's init function, parent::init() should be called before any other method calls.
Merge pull request #3083 from mparkhill/patch-1

FIX: parent::init is not called first
Merge pull request #3070 from dhensby/patch-3

Oembed warning if GD function is missing