Add `batch-hide-comments` feature #2988

Hiding 1 comment requires 3 clicks. If you have to hide more than one comment at once it can take a while. What if we showed some checkboxes on alt-click (or extra UI in the dropdown)

<details> <summary>Partial demo code</summary>

$$('.timeline-comment-actions > :last-child').forEach(menu => {
    menu.insertAdjacentHTML('beforeBegin', '<input type=checkbox class=ml-2>'); 
    menu.remove()
})

<div class="timeline-comment-actions flex-shrink-0">
    <button class="btn btn-sm btn-primary ml-2">Hide selected</button>
    <button class="btn btn-sm ml-2">Cancel</button>
</div>

</details>

<img width="210" alt="menu" src="https://user-images.githubusercontent.com/1402241/78917185-d6729680-7a8e-11ea-827a-d273c22f2f2b.png" align="left">

<img width="269" alt="batch hide" src="https://user-images.githubusercontent.com/1402241/78918814-441fc200-7a91-11ea-9296-0ac267358fd0.png" align=right>