add multiple media links for presentations

[elmiko]

This change adds the feature for having multiple presentation urls in a presentation entry.

changes

• update community template to process multiple urls
• update community template to add some spacing between links
• update all presentations with new link format
• update presentation template

[elmiko]

inspired by suggestions in #277

[erikerlandson]

This seems fine to me - is there an easy way to handle two cases: a single url or an array? that would make the pr much smaller

[elmiko]

i'll look into it, although i seem to recall that this didn't work before and putting some sort of type detection logic in the template just seems like overkill

[willb]

In general I can see a case for multiple media attachments but not necessarily to solve this problem. Why not just add the one talk we have with two media links as two “talks” instead of changing the template?

[elmiko]

ok, so it looks like liquid will handle the links properly if we have optional lists

@willb that works too, i'm good either way

[elmiko]

ok, updated so that it doesn't change all the preso. this will allow for option multi-links

[erikerlandson]

LGTM