General UI/UX improvements

[ellisonbg]

I went through most of the UI/UX and improved the behavior and styling. Some highlights:

• Better handling of dark theme.
• Shift+Enter keyboard shortcut for submitting the chat.
• Use JupyterLab default markdown styling.
• Better spacing
• Basic layout restoration, default location is L panel.
• New chat icon.

Here is the current state:

[3coins]

@ellisonbg
Thanks for making these changes. Shift + Enter works great, and input box looks very nice.

The background color in light mode does not have a good contrast, also it differs in how editor in notebook and markdown preview shows code blocks. Code blocks don't have any background in the editor; should we just add a minimal border around the code block to separate it? Another option might be to put a gray background, like we have for the cell block.

I would also decrease the spacing between the code block and Copy To Clipboard to maintain context.
With the panel now moved to the left, I could not see the file browser and chat open at the same time, so could not see the directory names when I wanted to index.

[ellisonbg]

@3coins yep, good feedback on the code block styling. I want to wait for a separate PR to make those changes. We have new UI mocks coming for that part and the styling is going to be a bit different as we are using react markdown (I think an extension of it) for that part. I will open up a follow on issue for that.

[dlqqq]

Awesome work!