Resolve cropped focus ring on Activity subscribe button

Problem to solve

As originally noted in !105918 (comment 1197348681), the focus ring for the subscribe button gets cut off on the right edge as a result of overflow: hidden being set on the parent container.

This appears to affect two locations: /dashboard/\_activities.html.haml and /groups/\_activities.html.haml

Proposal

Modify the container behavior to prevent clipping.