Merge pull request #229 from edx/christina/activation_email

Don't mention edge in the e-mail; same text used for edge and edx.

cms/templates/emails/activation_email.txt

-Thank you for signing up for edX edge! To activate your account, 
+Thank you for signing up for edX Studio! To activate your account,
 please copy and paste this address into your web browser's
 address bar: