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.
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.
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...
Adding AdminAssets filter Behat tests
Updated translations