Integration mattemost inside global chat tab

[svaroqui]

The feature consist of having a direct chat inside the product base on the mattermost OpenID integration

User connect on cloud18 via public instance aka https://dbaas-fr-1.signal18.io/ and register with it's gitlab account
At the login step we will use credentials to log to mattermost via openid and store token within the back so that proxying mattermost api is already login

The chat should comme with a the following item list:

• Private channels list of the user
• All Public channels
• All direct messages