Change RobotoSlab font to webfont (version v7)

[jessetan]

This PR:

• Adds additional formats (woff2, woff, eot) to the existing ttf format
• The woff(2) files are about 30%-50% of the size of the ttf files, which decreases time to download
• Supports modern browsers through woff(2), IE < 9 through eot, older Safari/Android through ttf. Unused font files are not downloaded by the browser.
• Uses the latest version of the font (v7) generated using https://github.com/majodev/google-webfonts-helper
• Does not check for local installs of Roboto Slab. This can be added at the slight risk of making the font look different. Users with the font installed will not need to download the webfont.
• Does not use character subsetting. Much smaller files are possible if only the Latin set is required

Related: #639