User/Person models merge

Merging of User and Person models. 
Following issue https://github.com/openSUSE/osem/issues/22

Migrations will move data from 'people' table to 'users' table (person.public_name => user.name, person.company => user.affiliation, person.irc_nickname => user.nickname, biography,  and all avatar-related fields). 

Tables 'people' and 'event_people' will then be deleted. Table 'event_users' will be created.

Everything that was associated with person will now be associated with user, necessary functions are also moved to 'User' model. 
Use def with parentheses when there are parameters.