More graceful handling of missing GET data in ModelAdmin

See https://github.com/silverstripe/silverstripe-cms/pull/253
BUG Consistently use FormResponse in CMS JavaScript (fixes #8036)

Regression caused by a security fix in 9bf3ae9a190
Fixed composer.json dependencies
Merge remote-tracking branch 'origin/2.4' into post-2.4

Conflicts:
	composer.json
	javascript/CMSMain_left.js
	javascript/LeftAndMain.js