Media grid:

* Introduce the concept of bulk editing via a separate mode. Like the list view, only bulk deleting is available. The UI is a little funky, especially with the field display toggles there, but refinements will come.
* Up the max thumbnail size from 120px to 150px.
* Slide-down panel for the add new uploader. Known issue: it doesn't close again.
* Remove the toolbar region in the EditAttachment frame.
* Defer a function call so the grid fills available space.
* Give feedback when no results are found. Also needs styling.

props ericlewis. see #24716.

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