Change the story license immediately

In the publish/share view of the builder, the story license (and the
Creative Commons image and link) change as soon as the radio buttons are
changed, eliminating the need for a submit button.

In order to make this work, the Story needs to have a default license value.
The Story model has been updated so that all stories have a 'CC BY' license
when instantiated.

I also removed the padding on the forms in the Publish/Share view, made the
intermediate text (the text shown before the CC image and link are loaded)
indicate that more data is being loaded, and added a fallback message for
when the request to the CC API fails.

Closes #517.
Fix uneven number of columns in the story list
Order "My Stories" list by last edited date

Order the "My Stories" list by reverse chronological date of last edit.

Closes #549
Add style for disabled buttons

Sets opacity to 65% for buttons and pseudo-buttons (elements with the
.button class) with the disabled attribute set (or .disabled set in the
case of psuedo-buttons).

Closes #516
Truncate non-news featured item excerpts

Truncate non-news featured items to 75 words.  I opted not to include a
"Read more" link because that would be redundant to the title and image
links.  Also, it's messy to check whether the text has been truncated or
not at the template layer and show the link conditionally.

Closes #589
Pull from piton/develop. Fix merge conflict in featured_object template.