Introduced (organizational) units

- New table "unit"
- Each "area" refers to one unit
- "invite_code"s are valid for one or many units
- New table "privilege" stores "voting_right" for units
- Delegation scope 'global' is replaced by delegation scope 'unit'
- View "global_delegation" is replaced by "unit_delegation"
- Removed helper view "active_delegation"
- Checking that members have "voting_right" in several views and functions
- New view "unit_member_count" to update "member_count" column of new unit table
- New argument list for function "delegation_chain"(...) containing "unit_id"
- Renamed column "member_active" to "member_valid" in "delegation_chain_row"
- Modified demo.sql and init.sql to support units