Support talks with multiple speakers

[AndrewVSutherland]

It is not that uncommon to have talks with two speakers (possibly more, but this is rare). We would need to work out the details of the UI and schema changes that would be required, but I prescribe this here; the main point is that I think we need to support this (eventually).

[AndrewVSutherland]

There are two possible ways to handle this in the schema; the "right" way is to add a talk_speakers table, comparable to seminar_organizers. But given how infrequent it is, it might also be reasonable to just make speaker, speaker_affiliation, speaker_homepage, and speaker_email arrays. We want to be careful not to do anything that will slow down the loading of our front page and forcing a join to get the speakers might do that, so perhaps an array is better.

For the UI, on the Edit talk page we could either do something similar to the seminar organizers table, or we could instead only show one speaker at a time and provide add/next/prev buttons to page through speakers. We should probably leave the Edit schedule page as is and assume just the details for the first speaker are entered there. On the main browse page I would just truncate the list displayed in the speaker column if it is too long to fit and add an "et al." at the end.

[AndrewVSutherland]

From @rbommel #555:

In my Google calendar, talks with multiple speakers do not look nice. E.g., for the "Panel discussion on hosting virtual events" the title of the panel has become "<a href="https://juliettebruce.github." in my Google calendar.