Add some plugin code to this plugin

[remcotolsma]

To simplify development, I think it is more convenient to include code from the following repositories in this plugin.

• https://github.com/pronamic/wp-orbis-projects/
• https://github.com/pronamic/wp-orbis-companies/
• https://github.com/pronamic/wp-orbis-subscriptions/
• https://github.com/pronamic/wp-orbis-timesheets/

[remcotolsma]

For now we decided to modernize the repositories a bit and look at this issue again from there.