Set up wiki-default image size

This commit makes several adjustments to make sure default size is being
handled correctly:

* Add wiki's default size configuration parameter to the
  VisualEditor.hooks.php file so it can be called from VE.
* Make sure new images are inserted with default size and are
  marked 'defaultSize = true' for the DM to handle.
* Force default size if 'defaultSize=true' in the DM
* Add a 'default|custom' switch to the media edit dialog for size
  inputs. When 'default' is chosen, the media size widget will be
  emptied, displaying its placeholders (default size)
* When the size widget's values are 0 it will automatically turn to
  default size values. If the value started default and the user
  typed in a size, it will automatically override default and use
  custom size.

Bug: 47804
Change-Id: Ib973ea2afa96090a4ba61b2b55ee63457f1329c1
Merge "Set up wiki-default image size"