Remove dependency on "welcome.md"

[mattstratton]

This path is a dependency from the old Webby site. New events don't require it and we can/should make sure we aren't looking for it in any code.

[bridgetkromhout]

I'm confused here because webby didn't use "welcome.md".

[mattstratton]

No, it didn't, but with how we did the migration there was a reason. I don't recall what it was, but it's not important why I guess :)

[mattstratton]

Related to #59

[mattstratton]

I am pretty sure this is handled, as the new index.md files are supposed to alias to /welcome so anything that points to it will work.