change with_billable_hours scope to method and query LEFT OUTER JOIN
change with_billable_hours scope to method and query LEFT OUTER JOIN

- convert the scope into method, in order to make use of other model methods
- LEFT OUTER JOIN query, in order to be able to get full query features
- add ARRAY_AGG() functions to select, in order to have hour.id and hour.meeting_date
  in an array within the query result
- exclude volunteers who allready have a billing expense in the given period

panter/aoz-003,panter/aoz-003,panter/aoz-003