Support chat bot widget

[hsluoyz]

Casibase can be embedded into another website as a widget in the page corner, visitors can chat with Casibase chat bot in the small window. Some examples are like:

• https://github.com/RocketChat/Rocket.Chat
• https://papercups.io/
• https://www.chatwoot.com/

It includes:

• A small widget button floating in the page corner.
• A small popup window (which shows the page: https://demo.casibase.com/) can be popped up when clicking the widget
• The popup window has a button to let the user jump to full bot page (aka: https://demo.casibase.com/)

We can first add the example code directly to Casdoor frontend: https://github.com/casdoor/casdoor/tree/master/web/src

But finally we will have a standalone JS to let people integrate it. Given this is a standalone JS, it should not rely on any heavy-weight UI library like Antd.