Add support for externally hosted calendars

• Use a different compile path for local and remotely stored calendars
• Render external links on the calendar download page, but use internal links for JS calendar displayt
• Add Pandas's calendar

@melissawm This is the alternative pathway I had in mind for https://github.com/scientific-python/yaml2ics/pull/71 and https://github.com/scientific-python/scientific-python.org/issues/246

The one problem with this approach (and any approach, in fact) is that our calendar rendering won't update when pandas changes their ICS. I tried to make the widget render from the remote URL, but that causes CORS problems.

Given that their calendar may frequently be out of date, does it make sense to host it here—or at least, include it in the calendar widget? Is there a reason why they don't want to use the YAML infrastructure provided?

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Since these calendars will get out of date (we don't rebuild every time they update), and since we can't render remote calendars, I think we unfortunately cannot support externally hosted Google calendars.