Implement 'Short Description' field on posts

Adding this to the meta data of a post will allow for authors to specifically write a more apt preview for their post.

• Allow input of short description when making a post
• Use short description in preview of post on condenser when viewing posts (instead of just grabbing the first x amount of text)
• Use short description when sharing a post on other sites (site previews)
• Default to first amount of text when short description is not given