Facebook dashicon for wp-admin

Add a new custom icon font containing a single glyph: vector Facebook logo. Display the Facebook icon using the `facebookdashicons` font character `E000` in the Facebook admin menu item as well as any class in the administrative interface with a `facebook-icon` class.

Removes Facebook icons previously used as background images, replacing with the font character.

Font will inherit the hover and active state fills from administrative theme CSS.