Refactor workflow

Hello Melvin,
i refactored the page workflows. What i did in detail:
- removed page_workflows model and table and wrote a migration to transfer this data back into the now extended comfy_cms_pages table
- integrated aasm (https://github.com/aasm/aasm) to add a state machine to make these workflows more readable, extensible and better testable. This gem also comes with a lot of handy extensions to the model including methods for checking the state (like page.published?) and automatic scopes (like @site.pages.drafted).
- changed the dashboard to use those automatic scopes
- wrote tests for page workflows to beeing able to refactor these in the future.
- changed the way workflows are handled on the site. Please take a look at this, i hope that you are ok with the way it works now.

I hope you like those changes and i am looking forward to seeing this merged into your gem.
Best,
Moritz