Placeholder talks should be able to contain other talks

[osfameron]

e.g. for Lightning talks such as http://act.yapc.eu/ye2014/talk/5526

(discovered because it turns out that the survey code doesn't send out evaluations for talks without a room. By being formally "contained" within another talk, the LTs should inherit the room? though I assume this original problem will get fixed within the survey code)

[osfameron]

@book points out "talk:$id" notation. Unfortunately this doesn't create a backlink from the LT to the container.
(And doesn't set room, but I think that's ok.)

[book]

And given that this notation can be used in multiple talks (and other places, including at least talks, events, and the wiki), nothing in theory prevents the link to be used several times, thus creating ambiguity as to which of the talks containing a link to a specific talk is the actual container talk.