Auto-populate minutes link for group calendar events

[mcking65]

Describe the issue

For events associated with a WG, TF, or CG, the calendar system could pre-populate the minutes link.

To reproduce

1. Create a new event on the calendar of a WG, TF, or CG.
2. Observe the field for link to minutes; it is empty.

Expected behavior

The system uses the short name of the associated group and the date of the event to construct a URI for the minutes.

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Additional context

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