Add online_url for Zoom and livestream links

[edalzell]

Summary

• Declare optional online_url on the event fieldset (Location section)
• Resolve ICS URL: as online_url → deprecated link → deprecated URL-valued location
• Document the new field and deprecations (fallbacks removed in 7.0 / Add nested location group; remove address, link, and sniffing #195)

Non-breaking / additive — targets 6.x so current sites can use join links without waiting for 7.0.

Closes #193
Part of #190

Test plan

• vendor/bin/pest --filter=IcsControllerTest
• Create an event with only online_url and confirm ICS has URL:
• With both online_url and link, confirm online_url wins
• Confirm existing link / URL-location behaviour still works

[edalzell]

Follow-up: kept the protected method named `eventUrl()` instead of renaming to `icsUrl()`. Renaming a protected method is a breaking change for subclasses, so that waits for 7.0 (#195). Plan updated on #190 / #193.

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