An option to add new users to Rosetta sites without sending an invitation

[petya]

The process of adding new translation editors per project will get much easier and less time consuming for GTEs if there was an option to directly add users to the sites instead of sending invites and waiting for confirmation.

Currently we can only send per project translation editors an invite, which means we need to wait and check if they've accepted to add them as per project translation editors.

This will become extremely heavy when the bulk of the requests from plugin authors gets in and I don't see how GTEs can keep track of who accepted their invite and when.

I think we shouldn't ask that of GTEs, let's make the process of adding PTEs a one step process to keep things simple.

Thoughts?

[ocean90]

We already have #1237 for this.