Jetpack Cloud: Add backup search view to backups page #3204652

Changes proposed in this Pull Request

• When a date range is selected on the backup page, switch to the “Activity Log view” and show a list of appropriate events, along with working filtering and pagination.
• Break out ActivityCard into it’s own component, retrofit into BackupDelta and use in the Activity Log view of the Backup page.
• A few misc improvements/fixes/advancements over a few other related areas.

As usual, this is not pretty, but in theory it should be fully functional.

Testing instructions

• View the backups page as a daily site, ensure the view still works as expected.
• View the backups page as a realtime site, ensure the view still works as expected.
• Click the calendar icon at the top of the backups page, select a date range. Ensure that the proper data is displayed, that further filtering works, that pagination works, and that you are able to exit back to the backups page view by clearing filters.