#68 Preserve chat messages and chat window status by using session store.

JavaScript:
- chat.js: General code improvements, add methods getChatStorage(), addToChatStorage(), setChatStorageWindowStatus(), appendChatContent(), blinkChatWindow() and loadPreviousChats()
- websocket.js: Add beforeunload event listener to close the socket

Templates:
- main.scala.html: Add loading of previous chats

CSS:
- htwplus.css: Add style information