Stats: Refactor various highlight cards into one component

[a8ck3n]

Currently have three different implementations of the highlight cards.

1. https://github.com/Automattic/wp-calypso/tree/trunk/packages/components/src/highlight-cards
2. https://github.com/Automattic/wp-calypso/tree/trunk/client/my-sites/stats/stats-email-top-row
3. https://github.com/Automattic/wp-calypso/tree/trunk/client/my-sites/stats/wordads

The general format is icon, title, and a display value. It might make sense to have the first two be props and the third be presented by the caller (ie: <Card title='Clicks' icon='arrow'>{ /* child elements */ }</Card>.

[jsnmoon]

A couple of additional refactors I think we could do:

• Refactor the count comparison card to reuse the count card (or the base card).
• Make the heading and icon properties optional for the count and count comparison cards.
• Refactor percentCalculator out of the count comparison card (maybe?)
• Refactor the count card to include tooltip functionality, which can be disabled via component properties.
• Deduplicate highlight card styling spread across the Stats codebase.

Two outstanding tasks @a8ck3n noted in #75989:

• Apply a min-width to each count comparison card so that the relative count doesn't overflow to the next line.
• Refactor annual highlight cards to encapsulate all render-related configuration to a single component.