Add link to story detail page from viewer

Add a link to the story detail page in the header of the story viewer.
This is particularly important for when the viewer is shown in a popup
from a widget.

Give the share link the class ``storybase-share-link`` so we don't
have to override the ``storybase-share-button`` styles and so it's
easier to match the styles of the other links in the ``#tools``
container.

Addresses #851
Fix summary layout on smaller displays

With a 50% column width, the table of contents overlapped the image.
Make the column with 48% to prevent this.

Tall featured images were also pushing the summary text outside of view.
Move the summary text to the top of the display, pushing the table of
contents down.

Add padding to the bottom or elements so they don't get cut off by
the footer.

Remove the byline as it's redundant with the omnipresent header bar.

Addresses #851