[Refactoring] Activity providers

Implements [ticket 3329](https://www.openproject.org/work_packages/3329)

:exclamation: is related to [openproject-meetings](https://github.com/finnlabs/openproject-meeting/pull/33)
:exclamation: is related to [openproject-costs](https://github.com/finnlabs/openproject-costs/pull/71)