Add focus outline to chatbot

[rebeccaalpert]

We should make sure that there's a focus outline in fullscreen and default and embedded. (Docked is fine.) This seems tied to the child components - I wonder if we can refactor a little so this won't be an issue.

Culprit seems to be the MessageBox without the history drawer, specifically

.pf-chatbot__content {
    position: relative; } 

position: relative; on the header also seems like a problem child.

Drawer is also a problem child.

See #296 (review).

[rebeccaalpert]

Fixed as part of initial PR - took a break and a solution came to me. :)