Add "user:invite" command

[mbrodala]

Similar to the web UI a user:invite command would be useful to trigger the regular user invitation workflow from the console.

This way no password must be set for a new user in advance, this should be left to the user in case he accepts the invitation.

Technically Piwik\Plugins\UsersManager\API::inviteUser() exists for this very purpose. Unfortunately there are 1-2 caveats with this API:

1. It is currently tailored for being called in a web context as indicated by the check for force_api_session:
   https://github.com/matomo-org/matomo/blob/5.1.2/plugins/UsersManager/API.php#L789
2. In case no "API session" should be used, it will prompt for the password of the current user:
   https://github.com/matomo-org/matomo/blob/5.1.2/plugins/UsersManager/API.php#L790
   This among others calls Piwik::doesUserRequirePasswordConfirmation():
   https://github.com/matomo-org/matomo/blob/5.1.2/core/Piwik.php#L297

Slightly hack-ish but probably the easiest workaround for now would be setting $_GET['force_api_session'] = 1; before calling ::inviteUser():
https://github.com/matomo-org/matomo/blob/5.1.2/plugins/UsersManager/tests/Integration/APITest.php#L1128

[mbrodala]

Evidently I misunderstood the force_api_session part. Not setting anything and having the default 0 (false) being used actually avoids the password confirmation.