PCIO VIS Boat

Adds a Boat Register to PCIO VIS Member Event for sailing clubs and other
boat-oriented organisations.

Features

• Register boats with: home harbour, name, type, model, build year, sail
  number and MMSI number.
• Link each boat to a registered owner member and zero or more crew members.
• Crew members can optionally be linked to a pcio-vis-products subscription so
  their activity status is shown next to them in the register.
• When a core member is deleted all their owned boats (and crew associations)
  are cleaned up automatically.
• Boat Register admin page at /vis/boats, access-controlled by the new
  vis_boat_manage capability (granted to Editor and Administrator on
  activation).
• Boat name(s) appear as a read-only column in the core member list.

This plugin stores all data locally in two custom database tables; it does not
communicate with any external service.