Update our license to AGPL v3

🎉 Yesterday we decided to change our software licenses to AGPL v3.

We will adopt the AGPL v3 (without any modification) for Twake. It concerns 100% of the source code of the software.
This approach is part of our plan to clarify LINAGORA's positioning and also to facilitate stronger links with our ecosystem.

Author: RomaricMourgues