Media: Check that a router region exists before attempting to access its child views. Add relevant documentation.

Merges [23260] to the 3.5 branch.

props koopersmith.
fixes #22930.
Limit the execution of wp_enqueue_media() to once per page. Prevents multiple _wpMediaViewsL10n variables.

Merges [23214] to the 3.5 branch.
fixes #22843.
Media: Automatically select uploading attachments in single-select workflows.

The code now automatically selects uploading attachments in all workflows. In a workflow that selects multiple attachments, all attachments added to the upload queue are selected. In a workflow that selects a single attachment (e.g. selecting a featured image), the last attachment added to the upload queue is selected.

Merges [23259] to the 3.5 branch.

props koopersmith.
fixes #22817.
Ensure that get_archive_template() only loads a post type archive (archive-$post_type.php file) if there is exactly one post type in the query.

Merges [23249] to the 3.5 branch.

props scribu.
see #22956.
Media: Do not update attachment display user settings when a single attachment is inserted.

This is consistent with 3.4 behavior. (User settings are now only updated when the user makes an explicit change.)

Merges [23261] to the 3.5 branch.

props koopersmith.
fixes #22841.
Media: Restore 3.4 behavior by consulting the old-school DB options for default align, size, and link properties.

This restores linking to media files as the default, over attachment pages. This 'default' cannot currently be changed by a user setting (per 3.4 behavior), due to the default database schema.

Merges [23262] to the 3.5 branch.
fixes #22841.
RTL fixes for internal linking. props SergeyBiryukov, helen.

merges [23226] to the 3.5 branch.
fixes #22835.
Restore TinyMCE dropdowns to their 3.4 appearance in RTL.

Merges [23275] to the 3.5 branch.
props helen.
fixes #23101.
Taxonomy postbox: fix toggling with "Choose from the most used tags" when several postboxes are shown, props SergeyBiryukov, fixes #23112 for 3.5