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. 
Explicit non-nil checks are usually redundant.