Blocks: Split frontend block styles into block.scss files

[youknowriad]

This PR builds on top of #1418 and splits all the blocks' styles into two files

• block.scss for styles shared with the frontend
• style.scss for styles only loaded in the editor.

This should allow themes to support the gutenberg blocks without any change. The styles can be overriden by themes.

Testing instructions

Try the following blocks and ensure they're showing up properly on the frontend and the backend:

• button
• cover image
• pullquote
• quote
• separator

[mtias]

just a note, we should clean these classes and make them proper modifiers.

[mtias]

We should make editable have a wp-blocks-editable class

[youknowriad]

This was an error I should move this back to style.scss and use > p instead for frontend.

[mtias]

More specific styles needed for:

We should make the dash in quotes not be absolutely positioned:

[mtias]

We also need to get the object-fit styles from galleries that was merged earlier today.

[youknowriad]

@mtias the is-cropped className was missing in the save function. Added in 705c835

[mtias]

Now the fixtures will hunt you...

[jasmussen]

Nice work!

I think we'll want to do a run on theme styles and polish all of these as a separate PR, but I think it would be great to get this in soon, especially since I did some polish work to many of the blocks in #1597.

[youknowriad]

Ok. I'll wait for the tests to pass and merge. :)