Add the ability to add links to external blog posts within my blog feed.
This is done based on a new `field_external_link` field that allows for
adding the external link URL and the domain name as the title.
The node links are then overridden to use the external link if there is
one, so the node title and 'read more' links are changed to use the
external link.
Currently, automated tweets are not generated for external posts.
Fixes#182
Check if a post has previously been sent to social media, by checking
the value of a `field_sent_to_social_media` field.
This field is hidden on the node add/edit forms, and populated when a
post is sent to social media. Once this happens, it will not be sent to
social media again.
This change also populates the field for all existing posts, so that
they won't be re-sent to social media either.
Oliver Davies