Convert :) to 🙂 instead of 😀

Inserting 😀 when :) is used doesn't give the message the same feeling as if it was simply :) inserted. The 😀 smiley is way too eager and could even be interpreted as if you are laughing at what people are saying instead of just signifying simple good feelings. Using the 🙂 smiley would be much more appropriate. As an example Apple also uses it in messages to convert :).

Closes https://gitlab.com/gitlab-org/gitter/webapp/issues/1469