Support grouped notifications

[connyduck]

https://docs.joinmastodon.org/methods/grouped_notifications/

[JPLeBreton]

One minor request for how this feature eventually gets implemented: give us an option to toggle whether the UI says "[person] and 15 others favorited..." or "[person] and several others favorited..." when more than 3 or 4 people engage with a post - effectively hiding the number under the word "several". Cohost did this and it became a deliberately appreciated feature, helping people further de-emphasize making numbers go up (something the rest of Tusky's UI already does a great job with!). Of course, seeing the number can be useful sometimes, so make it a toggle option.