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

Regression caused by https://github.com/silverstripe/silverstripe-framework/pull/2585
FIX: Prevent "Add Page" button being stuck 'focussed'
FIX: Prevent AssetAdmin duplicate confirmation for file deletion (fixes silverstripe/silverstripe-framework#2639)
Merge pull request #913 from kinglozzer/assetadmin-delete-confirmation

FIX: Prevent AssetAdmin duplicate confirmation for file deletion (fixes silverstripe/silverstripe-framework#2639)
Updated translations
Merge pull request #912 from kinglozzer/cms-page-add-button-focus

FIX: Prevent "Add Page" button being stuck 'focussed'
Updated translation masters
Fixing test failures because of Translatable and SiteTreeSubsites
Merge pull request #924 from halkyon/modules_illegal_extensions_tests

Fixing test failures because of Translatable and SiteTreeSubsites
Fixing test failures introduced by Translatable and SiteTreeSubsites

Same fix as e93abc6, except it applies to VirtualPageTest.
Merge pull request #926 from halkyon/virtualpage_extensions_fix

Fixing test failures introduced by Translatable and SiteTreeSubsites
Fixing test not auto-following admin/pages/SearchForm

With additional modules like Translatable, this test fails because
a 301 is issued to redirect the admin/pages/SearchForm request to the
correct URL containining a locale in the query string.
Merge pull request #927 from halkyon/cmsmainsearch_test_fix

Fixing test not auto-following admin/pages/SearchForm
Documented magic properties/methods of SiteTree and SiteConfig.
Merge pull request #932 from yearofthegus/document-magic

Documented magic properties/methods of SiteTree and SiteConfig.
Numerous PHPDoc fixes.
Merge pull request #935 from yearofthegus/doctag_fixes

Numerous PHPDoc fixes.
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: