#1 Optimize user-handling

By now, every new message is stored seperately in the
database - so its user is.

1. Edit script: Check if user already exists
The script should be optimized, so that it checks if
the author of a new message is already stored in the
database.

2. Optimize database
Further a new index has to be set: The primary key
should not be changed, because a combined PK of
incrementing number and username makes no sense. Though
there should be a new unique index created for the
usernames.