Split location into events::location fieldset

[edalzell]

Summary

• Move location + online_url into new events::location fieldset; events::event is schedule-only
• Drop type: section dividers from events::event
• Document Schedule + Location tab imports and the 7.0 breaking fieldset change
• Update test fixture blueprint (and Sites/events locally) to use two tabs

Part of #190

Test plan

• vendor/bin/pest
• CP: Schedule tab has recurrence/times; Location tab has place + online URL
• Importing only events::event no longer shows location fields (expected)

[cursor]

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