schema changes

• Global rename of 'Venue' to 'Assembly' as we're using it interchangibly anyway already
• additional fields in Assembly (formerly: Venue), Conference, Event and Room

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Assembly (formerly known as Venue):

• added is_official flag

Conference:

• add self_organized_sessions_assembly

Event:

• added kind: official/assembly/sos
• added assembly relationship
• added owner field (user for self-organized sessions)

Room:

• added description field
• new link type "media.ccc.de" (to use the vocplayer)