[AI] Add white-labeling tokens for the chat quick action button

[aagonzales]

The watson assistant team requested we help them support their white labeling needs. They want the chat quick action buttons to be disassociated from the core tokens (right now, we're using tokens like $link-primary and background tokens in the button. I've made a list of new tokens to add and just daisy-chained the values to our tokens for our themes but this will allow better control for white labeling. I did keep some core tokens in use for the more generic button states like skeleton and disabled.

Note: This is not a blocker for the watson assistant team, just a nice-to-have.

New tokens to add

New chat token	White theme	Gray 10 theme	Gray 90 theme	Gray 100 theme
$chat-button	$link-primary	$link-primary	$link-primary	$link-primary
$chat-button-hover	$background-hover	$background-hover	$background-hover	$background-hover
$chat-button-text-hover	`$link-primary-hover	`$link-primary-hover	`$link-primary-hover	`$link-primary-hover
$chat-button-active	$background-active	$background-active	$background-active	$background-active
$chat-button-selected	$background-selected	$background-selected	$background-selected	$background-selected
$chat-button-text-selected	$text-secondary	$text-secondary	$text-secondary	$text-secondary

Updated spec