Show the attachment area (context) even when it is empty

[basicthinker]

Is your feature request related to a problem? Please describe.

When new users start to use DevChat, they initially don't know they should first add code to DevChat as the context. Directly showing the attachment area (context) can help users understand the way to use DevChat.

Describe the solution you'd like

It can be similar to the following:

For DevChat, this area should be named Context instead of Working Set. We can move the model select and topic button below the input area and add the context above.