Plans m1: Show plans list

[koke]

First milestone towards Plans:

• Plans: Add plan item to site details #4747 Add a top-level Plan menu item (under Stats, to match Calypso) (@koke)
• Plans: Display user's current plan #4758 Retrieve the user’s current plan to determine what we show to them (@frosty)
• Plans: plans list #4772 Display a top-level list of Plans (@koke)

Mockups

Menu item:

Plans list:

[koke]

Uploaded updated mockups