The forum requires a magnolia-5-content-app which is the admin-interface to administrate forums (for sure to administrate the pages-comments-forum for the commenting-module, too).
For the beginning it's ok to provide the same fucntionalities which we had on magnolia 4.5 (and earlier) versions. See attached screen-shots.

Migration:
When doing this update, we also must care about migration of already existing forum-projects! (Not sure whether this requires a dedicated story-ticket.)

Note, that we received a contribution with a +/- working version which does to a certain degree what this ticket is requesting for. His version of the forum can be tried out with the branch "forum-5-migration". (see attached screenshots).
Further investigation is required to see what can be used and what not from the contributed version. What can be said until now:

• one screen is missing - the one which gives an overview of the mesages/threads which requires moderation (compare the screenshots)
• lacks migration, it only works when freshly installed.
• is using a lot of deprecated classe
• no i18n; labels, etc. is all hardocded in bootstraps

For further information update the migration of the forum (and commenting) to M5 see the wiki-concept-page: http://wiki.magnolia-cms.com/pages/viewpage.action?pageId=77463782