Update marketing/variations/schedule

Closes #411
Allow duplicates in the Sub header
update link for SPF as per Bryan Hammerquist
Merge pull request #427 from sendgrid/#411

Update marketing/variations/schedule
Update subuser/apps

Closes #422
add unsubscribes guide
updated subscription widget guide

closes #406
Update smtpapi/uniqueargs

Closes #383
change prod build to use same dynamic "version" as staging