Feature/prepare migration to meteor 3

[jbl2024]

See https://guide.meteor.com/prepare-meteor-3.0

• Use async everywhere
• Fork alanning:roles v1 to add async support (done, see https://github.com/jbl2024/meteor-roles/tree/v1)
• Remove meteor/simple-schema, and use simpl-schema provided by npm
• Use new veliov-files version with support for async (still pending, see Async support veliovgroup/Meteor-Files#865)