[Follow button] Update design and animate.

[alloy]

See https://github.com/artsy/eigen/issues/1132#issuecomment-213595688

[alloy]

@katarinabatina @briansw The animation is now in. Regarding the greyed out state when you visit an artist view of an artist you’re already following, I’m wondering why that actually looks different than after just following an artist, could you elaborate?

[katarinabatina]

@alloy while the following state in bright purple is good for communicating that the user has successfully made a follow, I think it's more distracting over time and the gray state allows the following to fade to the background once the user revisits the artist page later.

[sarahscott]

From what I can see, the 'Following' button is always bright purple whether you just followed an artist or are visiting the page of an already-followed artist. Should we add this grayed-out state?

(would be a Good First Issue)