Move external libraries from repository to composer

[NielsdeBlaauw]

First the easy ones. Some seem to have been edited, so it is necessary to check out if these edits break anything.

• sendgrid/sendgrid (^6.0)
• mailgun/mailgun-php (^2.3)
• google/apiclient (^2.0)

the mandrill-api-php library is probably harder, as it is on bitbucket and hasn't been maintained since 2015.

The Zend-1.12.10dependency is weird, but probably also isn't trivial to replace.