Remove unused Item::move().  To move an item, just change its parent_id and let
Model_Item and the item_moved event do the dirty work.  The only place I found
that references this function is the defunkt gwtorganize contrib module, so it
seems safe to go.
Rename Search::search_query_base() --> Search::get_search_query().
Rename Item::set_display_context_callback() --> Item::set_display_context() and
Item::clear_display_context_callback() --> Item::clear_display_context(), which
match Item::get_display_context().
Add @todo notes to User and Group classes - not sure if these errors still exist
Big rewrite of item/collection views (display context, siblings,
position, breadcrumbs, pagination, etc.)
- strip down Item::get_display_context() to just get context from cache.
- revise View_Gallery::init_collection() to use callbacks for the collection
query and breadcrumbs and to set the *same* callbacks as the item display
context.  This simplifies a lot of things...
- add View_Gallery::init_item(), analogous to View_Gallery::init_collection(),
which uses the display context and sets all sibling/position/pagination info.
- revise View_Theme::siblings() to match.
- strip down Breadcrumb::array_from_item_parents().
- add Breadcrumb::set_first_and_last().
- revise item collections (album, tag, search) to use new setup.  Among other
things, this removes a lot of position/sibling/display_context functions.
- revise non-item collections (users, comments) to use the new setup.
- revise the search breadcrumbs to include the "show" parameter as well as
have a more descriptive title (i.e. make it similar to the tag breadcrumbs).
Merge pull request #454 from shadlaws/fixes_20130711

Fixes 20130711
Make View_Gallery::init_item() & View_Gallery::init_collection() a bit quicker.
Net results are the same, but we're a bit smarter about when to run queries.
Merge pull request #455 from shadlaws/fixes_20130711_2

Make View_Gallery::init_item() & View_Gallery::init_collection() a bit quicker.
Make Gallery_View::init_item() a bit quicker yet.  Net result is still the same.
Merge pull request #456 from shadlaws/fixes_20130711_3

Make Gallery_View::init_item() a bit quicker yet.  Net result is still the same.
Fix use of item display context in Controller_G2 and Controller_ImageBlock.
Merge pull request #457 from shadlaws/fixes_20130711_4

Fix use of item display context in Controller_G2 and Controller_ImageBlock.
Revise View_Gallery item/collection view a bit.
- add View_Gallery::collection_type().
- revise View_Gallery::init_collection() to use it to detect item collections
more directly.
- revise naming of sibling/children/collection callbacks/queries - generic
things are called "collection," and once callback is run, called "children" in
collection context and "sibling" in item context.
remove redundant message block CSS class
Add multi-tag ability ("tag/slug_1,slug_2" finds items with both tags).
Make Controller_Tags::get_tag_query() quicker for single tag queries.
Merge pull request #459 from shadlaws/fixes_20130716_1

Fixes 20130716 1
Fix Rest_Data to ensure that cache control works.
Merge pull request #460 from shadlaws/fix_20130717

Fix Rest_Data to ensure that cache control works.
Merge pull request #458 from mstoltenburg/master

remove obsolete CSS class from upload form message-block
Resolves #2084