Drupal 8.x

How to print "Main navigation" in page.html.twig page in Drupal 8?
Please help.

Some problem: http://www.drupalcenter.de/node/55432

After replace core to new version from 8.1.3 to 8.1.5 have error:
PHP Fatal error: require(): Failed opening required '.../core/modules/user/src/UserServiceProvider.php' (include_path='.:/usr/local/zend-5.6/share/pear') in .../vendor/symfony/class-loader/ApcClassLoader.php on line 110

After I had read the documentation about the form/field api I wanted to create my first module.
My goal was to create a date field with a restriction, so that people can't set a date before tomorrow.
(the date restrictions module is not available for drupal 8)

The field is placed inside a contact form and the main goal is already working.
All the dates before tomorrow are restricted.
But I wonder if I maked some mistakes in my code so someone with more experience can review it.

While it's easy to change the error pages in Drupal I struggle to figure out how to deal with Symfony error pages.

Symfony deals with it like this: http://symfony.com/doc/current/cookbook/controller/error_pages.html but I'm not sure how to incorporate it in Drupal 8.

I want to avoid this:

Hi every one, there is something i don't get in drupal theming i think, if i create a content type lets say Bikes, i add 20 bikes, i create a view Bike-listing that list the 20 bikes, when i click on one of the bike (thru the content link i added in my view) it send me to the full content page of this bike but is there a way to also attach a custom view to this page ?

Thanks in advance.

I'm using a custom jQuery validation script as a custom Drupal.behavoir with a Drupal Contact Form on a Drupal 8 site which works fine if the user is not logged in, however when logged in the script will fire three times on submit. I assumed this was due an AJAX action that only present when logged in. I have the jquery.once dependency declared in my theme, and can see the jquery.once plugin in the compiled JS file, however I cannot get it to work at all. There are no errors in the console or in Drupal Watchdog, but jquery.once never adds a class to the context element.