Fixes to chatbot agent and page for chat app

[koverholt]

In the Vertex AI Conversation demo app, the existing chatbot in Dialogflow CX stopped responding to inputs/questions, seemingly it was using an old type of data store that was deprecated. This PR gets the demo app and chatbot back to working conditions via the following changes:

• Uses a new chatbot configured following the steps in https://codelabs.developers.google.com/codelabs/vertex-ai-conversation
• Refactors the JavaScript to run sample/demo queries due to upstream changes in DF Messenger JS client
• Adds a JS input event to make send button active after entering sample query text
• Adds a max-query-length parameter set to unlimited to work around a JS/state bug when sending follow-up queries
• Removed 37 HTML files from sample data set in GCS that were larger than 2.5 MB and exceeded the individual file size for the data store

Newly deployed (and working again!) chat app is available at https://vertex-ai-conversation.web.app/