Load more than 100 chat messages using a 'load more'-link

This adds a new message `GET_CHAT_MESSAGES`, which is now used the get the initial 100 messages, instead of just putting them into the `clientVars`. It also adds a link which sends the `GET_CHAT_MESSAGES`-message again to get the next 20 messages, until there are no more messages left.

I know that you want to fix issues before adding new features but this was marked as a bug, not as a feature request and I needed this functionality anyway.

Fixes https://github.com/ether/etherpad-lite/issues/337