Prevent flash messages from expanding offscreen

Prevent flash messages from expanding offscreen

Because we are using an edge-to-edge layout for the flash messages,
the negative margins on the row were expanding the element offscreen.

Bootstrap provides a "no-gutter" class for removing these margins,
but that also drops the spacing between columns which we want to preserve.
So instead we use a margin override on the row only.

Additionally, drop some padding css in favour of a utility class

Fixes #3351

Bug was introduced in 3dd639c8d02677b773935aa982ba0c9f3cc209e5