Add conversation-activity-filter feature - #4029

Closes #3764
Closes #615
Replaces #3847

Screenshot

(readme screenshot wip)

Test

Every event is on #4030
Some events are on #3847 but you can also test the native "X event hidden" there, which defers loading events in the middle

To test this feature, it's best to replace the CSS with:

:root:root .rgh-conversation-timeline-filtered {
	/* display: none !important; */
	outline: solid 5px red;
}

So that the elements are highlighted instead of hidden (which is harder to keep track of, mentally)

Example of the above CSS in "Show only unresolved comments" state:

Wishlist (new PR?)

• Move to header so it's always visible
• Ensure that the 2 instances are synced
• Hide resolved review comments that are among unresolved comments: Test-only PR with every event #4030 (comment)
• [/] Add "Events hidden, click to undo" message at the end of the conversation Since it's always visible in the header now, this is no longer needed