This pushes the
most important content in the history panel (the history items
themselves) much higher, and moves the clear history to just a button
next to the header (instead of a whole section).

| Before | After |
| - | - |
| <img width="1618" alt="Screenshot 2023-04-28 at 5 17 40 pm"
src="https://user-images.githubusercontent.com/153/235084313-6be3495f-72ee-4e97-953c-6f2921861f8e.png">
| <img width="1618" alt="Screenshot 2023-04-28 at 5 18 27 pm"
src="https://user-images.githubusercontent.com/153/235084369-6a6f2ac6-0e12-4b4c-981b-814aa7aa3e07.png">
|

Notes:

* In the process of battling the default button styles, I switched it to
use standard VSCodeButton elements, which give us a bunch of nice
defaults for free (hover etc).
* There's probably more work that can be done in follow up PRs to bring
it up to speed with #51072, and any new work in #51165, but this at
least improves what we have now.
* There was a keyboard handler and tabindex in there, but I couldn't
seem to get it to trigger. Would love help for adding it back in if it
was working.

## Future Fixes / Discussions

1. When starting your first chat, and then clicking history, I was
expecting it to appear in Chat History. But it's more like "Previous
Chat Sessions" — you have to reload, or leave and come back, to have new
things appear. I feel like the current live session could still be there
at the top of the History (if there's been a message sent), and clicking
it would just go back to it.
2. When clicking on a recipe, I thought it would start a new chat, and
the old chat would end up in history. But it just adds to the current
chat 🤔
3. If you use fixup, the chat messages (and therefore chat history
buttons) are a bit empty and awkward. <img width="322" alt="Screenshot
2023-04-28 at 5 49 23 pm"
src="https://user-images.githubusercontent.com/153/235087671-3e584ad2-0d8b-4f13-8c56-f0003b616414.png">
<img width="297" alt="Screenshot 2023-04-28 at 5 51 04 pm"
src="https://user-images.githubusercontent.com/153/235087983-76601551-cdbc-4723-94d2-4e56cc6a54ce.png"><br>
Perhaps it could say "I've updated your code." instead of "Here is my
attempt at rewriting the selected code:"
4. Sometimes the history buttons contain escaped HTML, which would
normally be rendered as HTML in the normal chat. It might be nice to fix
that.

## Test plan

Would appreciate another person’s manual test of this, given it's my
first contribution to the extension 🙏🏼

* With no history, open Chat History view, verify it works okay, and
that Clear History button is disabled
* Perform a chat, reload the window, open Chat History view, verify
buttons and labels
* Perform another chat, reload the window, open Chat History view,
verify buttons and labels
* Click Clear History