Properly Escape User's Name in PusherChatWidget.js

Hello All,

Recently, while I was working on my project Collab.Center (https://github.com/Mulletfingers999/Collab.Center), which uses Pusher, I found that a user's name was not properly escaped in the Chat (https://github.com/Mulletfingers999/Collab.Center/issues/7). I edited PusherChatWidget.js myself to fix this. Once I found Pusher was open sourced, I created a pull request. This pull request  properly Escapes `activity.actor.displayName`. Previously, if the user's name was 'Liam O'Flynn' it would become 'Liam O\'Flynn'.

Sincerely,
**Liam O'Flynn**