#4980336 Jetpack Scan: Show placeholders for list items while the threat history loads

Fixes p1HpG7-ab4-p2.

Changes proposed in this Pull Request

• Add necessary components to query the /scan/counts threat counts API endpoint in Calypso:
  • QueryJetpackScanThreatCounts query component
  • Action type constants
  • HTTP actions and handlers
  • Reducer functions and wiring
  • Selectors for the threat counts themselves, as well as the API request status
• Fetch counts of threats by status first, then show loading placeholders while full history information is retrieved
• Add pagination controls for long lists of threats
• Move the threat history list itself into a separate (directory-local) component, ListItems
• Move the threat history filter into its own component, ThreatStatusFilter
• Export new ThreatItemPlaceholder component to facilitate new loading behavior
• Add styles for pagination, loading scenarios, and scenarios where no threats exist for the selected filter
• Break long filenames in threat item headers so they don’t overflow
• Fix various linter and code style warnings

Testing instructions

NOTE: It will be much easier to test this PR if you have access to a site with no threats, a site with a handful of historical threats (< 10), and a site with more than 10 historical threats.

In Calypso and Jetpack Cloud, for both mobile and desktop layouts, verify the following:

Loading behavior:

• While the full threat history is loading, item placeholders are visible.
• The number of item placeholders roughly corresponds to the number of threats recorded for the site.
• If more than 10 threats will eventually be displayed, pagination placeholders are also visible.
• Loading placeholder styles appear correct.

Filtering and pagination behavior:

• The History page eventually loads all threat information correctly.
• Pagination controls are displayed if more than 10 threats match the selected filter.
• Switching pages works correctly.
• Changing the selected filter resets the current page to 1.

History item list behavior:

• Page elements and styles look correct after all information is loaded.
• Selecting a history filter where no results exist shows an appropriate message for that filter (e.g., “So far, there are no ignored threats on your site”).

Screenshots

A single page of threats

More than one page of threats (10 per page)