Use a different method for specifying an event homepage

[mattstratton]

Related to #47

Perhaps on the "welcome" pages, we add a new frontmatter tag, like homepage = "true", to specify that this is the homepage for an event and should get the extra love.

The other option is to create a new template called layouts/welcome/single.html and set the type of the welcome page to welcome in the frontmatter; this actually follows the convention of the other "special" pages (talk, program, speaker, etc) more closely.

This also will better preserve backwards-compatibility.

[mattstratton]

FWIW, I prefer the latter option (creating a new layout for "welcome" pages, and making the index page be of that type). Given that events are created using the script/sample event, we should be able to create a structure like this:

YYYY-CITY (directory)
- index.md (aliases to /events/YYYY-CITY/welcome/ and is of kind = welcome)
- location.md
- program (directory)
-- some_talk.md (is of kind = talk)
- program.md (is of kind = program)

Thoughts, @bridgetkromhout ?

[bridgetkromhout]

This is fine by me.