Dashboard storewall followup

What does this MR do?

• Display last storewall update on dashboard after refactoring from !1690 (merged)
• Fix an annoying display problem where 50px images are actually 75px big
• When confirming deletion, embed wallpost in scroll-area
  => the buttons remain visible regardless of post length

How confident are you it won't break things if deployed?

very

Links to related issues

• !1690 (merged) changed the dashboard behavior for new posts

How to test

have some stores and people that wrote messages on their walls. look at dashboard. see most recent entry for each store.

Checklist

• no unrelated changes
• added an entry to CHANGELOG.md