Only get twitter_username when needed (slideshow)

See https://github.com/storify/api/pull/396
Load story author identities from API instead of special case twitter_username
Merge pull request #29 from storify/story-perf

Load author identities from API
Fix for missing attribution href

http://storify.com/bigdrawmusic/big-draw-music-tour/grid
Merge pull request #30 from storify/fix/missing-attribution

Fix for missing attribution href
Fix for http://storify.com/wbur/photos-from-fenway-as-red-sox-world-series
Few CSS updates to make the slideshow look better for tweets with images in small size < 600px, e.g. twitter cards
Fixes [LF-8573]
Use matching linkify implementation to storify.com
Merge pull request #31 from storify/fix/certification

Fixes [LF-8573]
Dom based linkifyHTML for text elements
Merge branch 'master' of github.com:storify/templates into twitter-cert

Conflicts:
	slideshow/js/script.js
	utils.js
Use linkify on caption in grid template
Disable -webkit-filter in grid template because it doesn't work on retina devices

See https://bugs.webkit.org/show_bug.cgi?id=93471
Fix caption truncation
Make twitter logos link to twitter.com instead of permalink