Tweaks: "Hide following/mutuals indicators on notifications" option is broken on some items

[marcustyphoon]

Platform

MacOS 14.7.1 arm64

Browser

Chrome 131.0.6778.205

Addon version

v1.0.5

Details

The "Hide following/mutuals indicators on notifications" tweak doesn't appear to do anything. This seems to be because the activity CSS class we target isn't used any more; not sure what element it used to apply to and I can't be bothered to try to find out. is not present on some elements; see #1658. keyToCss('activity', 'activityItem') should work, I guess.

Assuming that we want this tweak and its friend "Make following/mutuals indicators on notifications more subtle" to be consistent in applying to activity and not applying to the community members page, I should also add the correct selector to #1654.