frontend: large project modification timeout fix
    
    On @rubygems/rubygems in Fedora Copr, we faced timeouts when we tried to
    disable mock chroots with too many BuildChroots.  Turns out ORM lazy
    loaded all the BuildChroot instances for disabled CoprChroot (which in
    our case was more than 100k items).
    
    Instead of this, prepare a custom query for related BuildChroots that
    can be used with limit() safely.
    
    For the purpose of the query, we need to use FINISHED_STATUSES variable
    which was improperly using states, not statuses.  So I'm fixing this
    (status is the number, state is the string human-readable alternative).
    
    Fixes: #1925
    Closes: #1968