Limbo state for talks

[andreasscherbaum]

The current talk selection process is missing a state for the talks: a talk which is selected by the CfP committee as potential candidate when one of the regular talks is not accepted by the speaker. Ideally a replacement at this point is not selected from the reserve list, but another speaker is given a chance.

This is currently handled by maintaining an external list, while the talk itself is kept in "submitted" state.

For the speaker it would be better that they know that the talk is still considered.

@xocolatl proposed the following text for the state change email:

Your talk <insert talk title here> is being considered for selection.  If you are unable to come, please withdraw; otherwise we will contact you later with our final decision.

The state change will be "submitted -> limbo", and from "limbo" to either "withdrawn", "rejected", or "approved".

And obviously this state needs a better name than "limbo".