Add completeness bar to workflow selection buttons

[rogerhutchings]

The design for the workflow select button in the Hero widget includes a completeness bar which needs to be added:

Related: #1077

And https://zooniverse.slack.com/archives/C0Z7YNTSL/p1554151344033200

[beckyrother]

(Working on this now)

[beckyrother]

Think I was trying to be too clever the first time. How does this feel?

For reference, here's the first one we were thinking about (pretend the button text is black like it should be):

[eatyourgreens]

Lose the blue tooltip, and I think this is much better (and something we can implement.)

[eatyourgreens]

#1204 adds a basic background gradient, which is muted somewhat on hover/focus. Each link is announced in VoiceOver as something like 'Link: Workflow name 60% complete'.

[beckyrother]

If we need to remove the blue tooltip, let's add a secondary line to the button to indicate the percent complete and give a reason that the button is a progress bar.

Note the white 1px border between the gradient and the box background.
Gradient: #F0B200 to #D46256