Moves get_content to Item instance method
Adds new item#tags assignment

Item method #tag_names plucks tag names and joins them with a specified
separator (space by default). Method #tag_names= splits a string by spaces,
then maps each element (a tag name) to Tag.first_or_create (using the item's
user_id as the new tag's user_id). Both #tag_names= and Tag's before_save
downcases the name.

Also moves get_content back to helper method because it relies on image_tag
(and would still rely on erb's html escape if it were tagged manually).
Conditionally appends ... to item text summary
Fixes broken conditional statement in item.rb