implement versioning for CMs content of activities

this MR implements version for the CMS-generated content for working groups.

Once someone clicks the Edit button on a detail page

the user is redirected to the current published version or a draft if existent. for the current published version, the user first has to create a New draft at the upper right

then the user can make changes with the usual CMS functionalities, and ones this is done, the user has to click Publish (again in the upper right) to display the changes on the Working Group Detail Page

@csteger don't know if you want to inform the CO-Working group about this change?

closes #196 (closed)