Tweaks: Fix subtle following/mutuals indicators option

[marcustyphoon]

Description

The "make following/mutuals indicators on notifications more subtle" tweak's purpose is to hide the text of the indicator in question, leaving only the smaller icon unless the user hovers over it. Staff removed the icons in question, breaking the feature; this adds the icons back and restores the functionality.

It also makes some tweaks required to work in the new activity notification types and to exclude the similar elements on the communities members page (as per the name of the feature; we could also make it work there but that seems like a separate thing).

Resolves #1649.

Improvement on #1568.

• handle new activity item format, addressing note in Tweaks: "Hide following/mutuals indicators on notifications" option is broken on some items #1692

Testing steps

• Confirm that the feature functions correctly on a new "replied to your post"/"replied to you in a post" notification from a mutual/followed user on both the activity page and in the activity modal.
• Confirm that the feature functions correctly on the classic style of notification (most other notification types) from a mutual/followed user on both the activity page and in the activity modal.
• Confirm that the mutual/following indicators on the communities members page are unaffected.

[marcustyphoon]

Well, this is a bit inconvenient. This stops happening if I set the display property on the random-div-containing-a-zero-width-space-for-wrapping-reasons to inline-block, I think, but t's not the most elegant thing to target with a CSS selector.

Edit: Ooh, it's even funnier without animations disabled :D Why is the svg span not being included in its parent's bounding box? I guess because it's itself inline-block?

Edit: Ah, it's because there's an anchor element covering this whole thing. I'm really not familiar with how pointer events work in those cases.

Edit 2: I think I see: the only reason that the blog name link is clickable/hoverable is because of a .activityContentWrapper a { position: relative; } rule, which causes the blog name link to create a new stacking context and I guess that punches through the overlaying anchor element. So making the mutual/following icon create a stacking context as well will allow it to punch through in the same way.