Add nested location group; remove address, link, and sniffing

[edalzell]

Summary

• Declare location as a group (name + nested coordinates) beside existing online_url
• ICS reads location.name / location.coordinates / online_url only — drops address, link, URL sniff, and top-level coordinates
• Replace protected icsAddress() / eventUrl() with icsLocation() / icsUrl()
• Docs + mapping table for the 7.0 contract; start upgrade notes (migrator in Add update script migrating location fields to 7.0 #196)

Closes #195
Part of #190

Blocked for Prime sites until transformstudios/prime#834.

Test plan

• vendor/bin/pest (142 passed)
• CP: Location group + Online URL on an event blueprint importing events::event
• Physical / online-only / hybrid ICS downloads match the mapping table
• Confirm legacy address / link / string location no longer appear in ICS