JC-1878 Add ability to create an admin user on first deployment

New migration file 'V61__Add_admin_user.sql' was added.

sample-forum.sql file was changed to deploy only registered,
banned and moderator users, because admin is created with first deploy.
Merge pull request #24 from jtalks-org/JC-1878

JC 1878 Admin user can be created on first deployment now.