Added sonata_page_content block to standard_layout.html.twig

Diff with [white space ignored](https://github.com/sonata-project/SonataAdminBundle/pull/1711/files?w=1)

My use case:
I need to create special side box with custom content on the left and do not break any other SonataAdmin functionality (calling `parent()` after extending block). 

This block will also enable easier overloading the whole content of page (the top menu will still be available)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -