Dashboard sections (#38463)

* Add `ADD_MANY_CARDS_TO_DASH` action

* Reuse `NewDashCardOpts` type

* Extract `DashboardCardLayoutAttrs` type

* Add basic `DashCardPlaceholder` viz

* Add section layouts

* Add `addSectionToDashboard` redux action

* Add `SectionLayoutPreview` component

* Add "Add section" button to dashboard header

* Allow placeholder cards in `replaceCard`

* Make heading cards transparent by default

* Fix layout fn usage

* Update mapping UI for placeholder cards

* Fix tooltips position

* Add `createMockPlaceholderDashboardCard`

* Test `replaceCard` works with placeholder cards

* Extract common `setup` from cards actions tests

* Add `createMockDashboardTab`

* Add unit tests for `addSectionToDashboard`

* Placeholder cards work for emails/pulses

Dashboards with placeholder cards now properly email rather than throwing an exception. Placeholder cards themselves are not sent as part of the email as they are empty.

* Merge remote-tracking branch 'origin/38209-dashboard-sections' into 38209-dashboard-sections

* Adding tests for placeholder cards and empty dashboards

* Fix parameter mappings UI

* Update section card sizes

* Clean up

* Add button label

* Add E2E test

* Revert

* Reorder

* Fix menu position

* Assert placeholder card doesn't have a button

* Fix menu tooltips

* Fixing unit tests

* Removed dangling code

* Fixing unit tests

* Remove virtual cards handling from `replaceCard`

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Co-authored-by: Mark Bastian <markbastian@gmail.com>