Styles: Remove edit-post styles from editor components

[aduth]

Related: #4813 (comment)

This pull request seeks to eliminate direct references to edit-post from editor stylesheets. edit-post is intended as an extension of the underlying editor components, so it is not appropriate for editor to circularly reference edit-post.

As part of these changes, the Warning component has been updated to...

• Not make assumptions about the type of content
• Not force implementer to apply specific markup structure, instead providing actions if button options are intended

Testing instructions:

Verify that there are no regressions in...

• Toolbar positioning for floated elements
• Warning displays (try invalidating a block by removing classes from the cover image markup in post-content.js)
  • Particularly: Font size, family, and button placement