AAG: add placeholder for cards that load external content

> After purchasing a backups plan and entering credentials, if I refresh the At A Glance page, the “Activity” card is slow to load and causes some reflow as it is added to the dom.
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> My guess is that we’re likely waiting for an API call to return before we show the card. Can we either cache that API call so the reflow doesn’t happen each time or show a pulsing/loading state for the card while the API request is in flight?

https://videopress.com/v/ZdRoq8ox

Reported here: p8oabR-qW-p2#comment-3597

Start by locating the At A Glance Activity card and the API request that loads its external content. Verify the refresh behavior, then add a loading placeholder or otherwise prevent the card from causing reflow while the request is in flight; done means the card has stable space before its content arrives.

Written by the indexing model from the issue text.