Reimplement breadcrumbs in site administration.

After discussion both inside and outside of Moodle it has been decided upon that we need to investigate and implement a back button that behaves in a similar fashion to the back button within the browser.

It would be preferable if we could store the anchor of the previous page so that when we navigate back we can set focus to the actioned item.

The back button should show on pages where a combination of primary & secondary navigation items does not allow the user to navigate back easily i.e. when on the admin settings page navigating to online text assignment you should be able to navigate back to admin settings > plugins rather than navigating back to the top page of site administration.

This may be of interest: https://developer.mozilla.org/en-US/docs/Web/API/History/back 

Cheers,

 For now we are going to put the breadcrumbs back for the site administration pages.
Things to do:

1. Items that exist in the primary and secondary navigation should be removed. e.g Site administration/Plugins. These are in the primary and secondary navigation respectively.
2. MDL-72427 has been reverted.
   So we want to check that the behaviour that we had in 3.11 is still present here.