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
API Make default gridfield paging configurable
Documentation improved
DOCS: Minor fixes to RestfulService documentation
Merge pull request #3026 from tractorcow/pulls/3.1-gridfield-paging

API Make default gridfield paging configurable
Fixed config setting link
Merge pull request #3076 from martinduparc/patch-4

Fixed config setting link
BUG Fix ImageTest
Image test would erroneously reset the Image::$backend to null if the test was skipped, breaking subsequent test cases
Merge pull request #3068 from kinglozzer/patch-1

DOCS: Minor fixes to RestfulService documentation
Removed demo folder
BUG Fix jstree when duplicating subtrees
Merge pull request #3031 from tractorcow/pulls/3.1-fix-tree-duplication

BUG Fix jstree when duplicating subtrees
Merge pull request #3078 from tractorcow/pulls/3.1-remove-thirdparty-demo

Removed demo folder
Merge pull request #3077 from tractorcow/pulls/fix-imagetest

BUG Fix ImageTest
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
FIX folder Title not being exactly the same as Name field if setName modified the value
Change PHP 5.6 to an allowed failure