Add partial rendering for user list

[galfert]

Similar to #92, opening a channel with a lot of users (e.g. #debian on Freenode) is slow, because it takes so long to render the list of users.

To improve this, we can display only a certain number of users at first, and then display more when scrolling to the bottom of the list. See #162 for the implementation using an InterectionObserver for the channel's messages.

[raucao]

I think you don't mean "pagination", but partial rendering.