Button updates

[megsfulton]

Update the styling of the X, Publish, and Done buttons.

These updates address the feedback we heard in Alpha 2 testing about the "done" button not being obvious. They also bring Android closer to the Kanvas UI.

What's changing:

1. Adding a black transparent circle behind the X icon (same as the other icon buttons on the editing screen)
2. Changing the "Next" button to a white circle with the Material arrow icon.
3. Replace the "done" text button with a white circle button with the Material check icon.

[aforcier]

@megsfulton what should the button be in the case where we're editing a story from Gutenberg (#445)? It's currently 'Done', should we use the same checkmark button as in the text editing view? Or perhaps a back arrow?

[megsfulton]

Let's use the checkmark there.

[aforcier]

Addressed in #563.