Register with Packagist

[rimas-kudelis]

https://plugins.roundcube.net/ has a short manual on how to publish a plugin with them. We should do that.
However, before doing that, the plugin should be renamed vexim_account_admin (with vendor vexim (see #16)).

[rimas-kudelis]

RC plugins are now to be registered on Packagist, so it's even easier.
I can take care of the registration should we ever fix it to work with recent versions of RC.