conversation-activity-filter - Turn into svelte component - #9675

This doesn't save bytes, but it helps keep the feature readable by extracting the large component's markup to a dedicated file.

For example it only now occurred to me that we're doing so much work on the page even when none of this is needed most of the time. I'll look into activating the comment observer only after the user picks an option from the menu.

select-notifications might be next if this is successful

😍

In the last copilot commit I took advantage of Svelte, which let me skip the raw CSS and related classes altogether. This sort of diff, paired with the readability of the resulting code, further justifies using svelte components.

The drawback is that this PR adds about 4KB to the final bundle. It's unfortunate that JSX is more efficient than Svelte in this regard.

To maintainers: Please add labels to this PR