FIX: moves calculateHeight after render (PR #520)

This will need more refactoring in the future, but it seems needs for now, most likely because of the previous line chat.Set("fullScreenChatOpen", true) which probably impacts other elements.

This commit is meant as a partial revert of FIX: simplify code when inserting full-page-chat · discourse/discourse-chat@19a57e0 · GitHub