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

Regression caused by https://github.com/silverstripe/silverstripe-framework/pull/2585
Page edit access with CMS_ACCESS_LeftAndMain (fixes #908)

"Access to all sections" implies edit access in the CMS
FIX: Changetracker causing 'saveButton is not a function' error

Regression from 1571d35
Fix “I can insert an image from the CMS file store”.
Fix behat tests for insert-an-image.feature

- Fix “I can insert an image from the CMS file store”.
- Remove unused scenario (we no longer support adding multiple images at once from the CMS).
- Fix “I can edit properties of an image before inserting it” scenario.
- Remove “I can edit dimensions of an image before inserting it” scenario - these fields no longer exist.
- Add documentation to explain why this scenario is marked @todo.
Fix behat tests for insert-a-link.feature

- Fix "I can link to a file" scenario.
- Fix "I can link to an anchor” scenario.
- Fix "I can edit a link” scenario.
- MINOR: Grammar fixes.
- Fix "I can remove a link” scenario.
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:
Minor improvement of theme switching test performance.