Important bugfix, affecting correct counting of supporters/votes

Changed "trustee_id" to "truster_id" in following views:
- global_delegation
- area_delegation
- issue_delegation

Also updated comments for that views.

The new behaviour was expected by the following functions:
- membership_weight_with_skipping
- weight_of_added_delegations_for_population_snapshot
- weight_of_added_delegations_for_interest_snapshot
- weight_of_added_vote_delegations

The bug caused inactive members to falsely increase voting weight of trustees.