choosing a random image is essentially a function on an item collection.
Also implemented a bunch of other query filters for item collections.
Created item::random_query() as a way of generating a reasonable
starting point for random queries.
* Extend block_manager to handle sidebar blocks. get_available has become get_available_admin_blocks, get_list becomes get_admin_list.
* Create new functions get_available_site_blocks which will look for gallery_block get_available_site_blocks.
* Refactor sidebar_blocks into a separate function and then call block_manager::get_html(site.sidebar). Convert image_block to use block management instead of theme::sidebar_blocks
* Change the block_manager api so that the theme is passed into the get method. convert info to the new sidebar block approach
* Convert the user module to use the new sidebar block structure. remove the installers for info and image_block modules.
* Convert tag and rss modules to the new sidebar framework. reset the version number to 1 for info and image_block modules.
* Change the get_html method to ignore empty blocks and change the individual handlers to return an empty string if no block is generated
* Add a warning message if no sidebar blocks are active and provide a link to the admin page that configures the sidebar.
