Add tip about 'required' in sonata_type_collection

Document 'required' option behavior in sonata_type_collection
Pass admin through to autocompleteItems callback:
~ modified HelperController::retrieveAutocompleteItemsAction() to pass $targetAdmin through to callback instead of $datagrid
~ updated documentation of the callback
Fixed an issue with one to many relations

As described in #2325
Merge pull request #2353 from caponica/autocomplete_callback_with_admin_v2

Pass admin through to autocompleteItems callback:
Merge pull request #2351 from eikes/master

Add tip about 'required' in sonata_type_collection
Fix css - added scrollbar on list view for small screens
Autocomplete changes:
Removed initSelection method in favour of directly setting the initial value
Added extra (not null) check autocompleteInput onChange function (prevents error when first called)
Merge pull request #2169 from antoinemineau/master

Fix css of list box on small screens (added scrollbar)
Merge pull request #2326 from caponica/autocomplete_refactor_init

Refactored how the autocomplete initialises its values
Minor improvements in translation messages in sonata_type_model_autocomplete
Merge pull request #2363 from pulzarraider/autocomplete_msg_excp_impr

Minor improvements in translation messages in sonata_type_model_autocomplete
Merge pull request #2358 from net02/patch-3

Fixed an issue with one to many relations
Fixed typo in tab menu template
Merge pull request #2373 from Bladrak/fix_typo

Fixed typo in tab menu template
Removed problematic overflow CSS rules for ".box .box-body" in layout.css
Merge pull request #2374 from pulzarraider/overflow_fix

Removed problematic overflow CSS rules for ".box .box-body" in layout.css
Merge remote-tracking branch 'origin/master'