Update terminology - Extensions en (#276)
Update terminology - Extensions en (#276)

* Update extensions terminology

Update terminology in extension development activity per new guidelines at https://wiki.mozilla.org/Add-ons/Terminology.

This also includes resources and documentation found on the wiki.

* Extensions - fixed grammar mistakes

Fixed grammar mistakes found in initial pull request (https://github.com/mozilla/activate.mozilla.community/pull/276#pullrequestreview-55038648). Thanks, @MichaelKohler. :)

* Remove 'about' from social media metrics

Line should now read: 50 social media posts (Twitter, Facebook, blogs, etc.) that reference extension development and #MozActivate.

MichaelKohler/activate.mozilla.community