Add user to StoryAttachment

Add migration to add user_id column to story_attachments table
Set user when creating StoryAttachment
Add english translations for story attachment deletion

Two new messages, one for successful story attachment deletion and one
for an error when deleting a story attachment.
Redirect /stories/attachment/:id to image url

When getting an attachment via a `/stories/attachment/:id(/:style)` URI
redirect to the url of the attachment image instead of serving it
directy.
Define `/stories/attachment/:id` as the attachment_url
Complete the `StoryAttachmentsController#destroy` method

Rename it from `#delete` to `#destroy` and map it in the routes.
Merge pull request #6 from bryanjswift/attachment-user

`StoryAttachment`s belong to the user creating them