Allow multiple CCLA invitations to be sent at once

:fork_and_knife: You may now specify a comma separated list of email addresses to invite contributors to a CCLA.
Also, I realized that the language is conflicting and potentially confusing. The `manage_contributors?` is intended to be used for the Invitations management. However, in the UI it is referred to as Contributors. Whatever is the most simple and makes sense to you works.