Added featured_image field.

Added a field to be used with featured images. This field will be using
"public" file system as all of those images should be public. This
should deal with cases when "private" file system is set to be default.
There is no default_image defined as there is none to be taken.
Added featured_image field instance to ed_news.

Added a semi-configured featured_image instance to ed_news node type.
Some of the configuration might still be missing as that would require a
decision on sizes (forced maximum file size to 5 MB). Field should be
available and configurable. Also fixed an unexpected behavior when
fields were gone missing as soon as last node having instances of
corresponding fields was removed. Calling the function from edidaktikum
install procedure to make sure any missing ones are added.
Merge branch 'master' of github.com:centre-for-educational-technology/edidaktikum