Use font-stack over installing web fonts

Now using Helvitica Neue/Arial and Georgia over Open Sans and
Merriweather to reduces the size of the application payload and increase
loading times. The fonts themselves were not adding much to the overall
experience.
Remove font adjustment on blockquotes
Remove placeholder on activation code input
Remove Google Font stylesheet
Tweak font stack for Ubuntu
Merge pull request #1476 from hypothesis/remove-web-fonts

Remove web fonts