Support for "nature of statement: expected" qualifiers on term dates about commons-builder

[alexdutton]

I don't think the membership can have an expected_start or expected_end, as that would be qualifiers-on-qualifiers by this model.

If the membership has a start date or end date then we can add that alongside the expected start and end dates without it necessarily being confusing.

If the membership and term both have non-expected (i.e. happened) end dates then we could copy the term end to the expected membership end, but I'm not sure about that — how would that interact with overlapping term modelling?

[ostephens]

If the membership already has an expected_start or expected_end property, what should the behaviour be?

• Always prefer the dates already on the membership
• Prefer the later expected_start and the earlier expected_end
• Something else ?

[ostephens]

I think if membership and term both have start or end dates that have already occurred we should always take the membership dates - as it will be a relatively common thing for a membership to start at a different time to the term (e.g. when people die, resign, get elected in a by-election, etc.)