BUG: Fix add new button not passing the correct parent ID.

The `%s` parent ID placeholder was being URL encoded, so it was not being
replaced properly when adding a new page. This fix separately adds
placeholder parameters to the URL to avoid this issue.

The fix is not ideal, but there is no easy way to indicate that some
URL parameters should be URL encoded and others should not while still
correctly constructing the URL.
Merge pull request #870 from ajshort/pull-1

BUG: Fix add new button not passing the correct parent ID.
Calculate where to scroll to based off the position in the div

Also, only scroll if not currently visible.
Merge pull request #871 from simonwelsh/3.1-scroll

Calculate where to scroll to based off the position in the div
Add an extension hook for customising site tree hints.
Merge pull request #873 from ajshort/pull-2

Add an extension hook for customising site tree hints.
Avoid encoded HTML breadcrumbs
FIX Remove invalid single quote
Merge pull request #876 from cieszak/3.1

FIX Remove invalid single quote
Behat travis tests
MINOR add hook for extensions to update current page id, before falling back to default
Merge pull request #879 from sheadawson/833-currentPageID

Add hook for extensions to update current page id, before falling ...
ContentControllerSearchExtensionTest doesn't clean up after itself.

The problem is FulltextSearchable::enable() does two things:
It enables the extension, as well as sets the MySQL create table option
to MyISAM. Later tests run with the extension enabled, because it doesn't
get removed at the end of the test, but the table type is set back to
InnoDB when the test is reset and schema recreated.

This produces side-effects where later tests in a suite tries to run
ALTER TABLE on File and add fulltext indexes when the table type
is set to InnoDB, causing an error.
Merge pull request #880 from halkyon/fulltextsearch_text

ContentControllerSearchExtensionTest doesn't clean up after itself.
Default CMSMain to Versioned::reading_stage("Stage")

Regression caused by https://github.com/silverstripe/silverstripe-framework/pull/2585