Fix inviting many users pushing invitation input offscreen

Previously, when inviting more than a handful of users at once, the invitation textbox would get pushed offscreen by all of the 'user was invited' messages. Now we only show the last 3 users invited to the channel.

I've limited the height of the invitee list to 250px and set it to scroll on overflow. Here's a screenshot of it in action.

Closes #1750 (closed)