Added keyboard shortcut to add-feed

* Used the BIG_FREE_READER shortcut of 'a' to add-feed.
* I only added the en.yml localization.
* I didn't see any tests for shortcuts, so didn't add one.
Updated keyboard shortcuts screenshot
Merge pull request #201 from queuebit/master

Add 'a' keyboard shortcut to add-feed
update translations
fix es.yml
Prevent n+1 queries when serializing stories to json
Merge pull request #202 from ejfinneran/perf-fix

Prevent n+1 queries when serializing stories to json