Default CMSMain to Versioned::reading_stage("Stage")

Regression caused by https://github.com/silverstripe/silverstripe-framework/pull/2585
Merge remote-tracking branch 'origin/3.0' into 3.1
FIX: Fixed testing of theme switching.

The previous theme test relied on the existence of the tutorial theme, which has been removed.
I'ved added a couple of behat rules for scaffolding fake themes, and altered the test to use these.
FIX: Added proper state-restoration code to the ThemeContext:
Minor improvement of theme switching test performance.
Merge pull request #937 from sminnee/fix-theme-test

FIX: Fixed testing of theme switching.
FIX Limit ExtraMeta to only allow meta and link elements
FIX JS issue when enabling save buttons for when the content has changed. Current implementation triggers a 'Object is not a function' error. New implementation follows the same logic as in the onmatch function, and uses .button() as the function
Merge pull request #945 from patbolo/save-buttons-status-issue

FIX JS issue when enabling save buttons for when the content has changed...
Updated translations
Merge pull request #943 from hafriedlander/fix/extrameta_whitelist

FIX Limit ExtraMeta to only allow meta and link elements