Adds favorite/unfavorite button to item#show

New routes added for item/:id/favorite, using methods post and delete.
Changes reminder/favorite thumbnail layout

The link now encompasses the entire list item, rather than just the
48px thumbnail in the corner.
Adds new formatting to favorites as well
Adds header nesting level to Markdown parser
Adjusted item edit pane
Moves a list item into a partial
Adds Share model and section on the main page

Shares are between an item and the user who is allowed to view it.
Adds validations/fitlers to Share & ItemController

Also hides edit controls for non-owner on item#view
Allows owner to view their own items again
Fixes broken association
Restricts reminder viewing to item owner
Adds shared_users association on Item

Also adds error flash to main page layout
Adds search!

User and Item have #search actions, which respond to GET requests.
Currently they just do a "where name ILIKE searchterm" query, which is
specific to Postgres, so it should be changed to something else soon.
(Not to mention it's a pretty terrible search method to begin with.)

Controller callbacks and layouts were tweaked to accomodate the new
actions.
Adds item thumbnails to item searches.

Also adds user links to user searches, but User doesn't have a show
view yet.
De-links user search results
Allows user to search their own items
Styles main#search view
Uses multi term searches
Adds padding to text-preview view
Adds missing space to item detail