Add many-to-many parent-child org relationship

This is done in the same way as Whitehall does, except for not
including timestamps on the organisational_relationships table.
Add inferred parent org to parent_organisations on import

This is still in addition to the original parent_id foreign key.
Display multiple parent orgs as a sentence of links
Update access control to allow for multiple parent orgs

A user can edit an organisation's sites if they are a member of one
of the organisation's parent organisations. A user can be a member of
only one organisation in Signon, so it is not possible for a user to
be a member of all of an organisation's parents if there are several.
Remove remaining uses of parent foreign key
Set many-to-many parent relationship from foreign key
Remove parent foreign key from organisations
Ensure that import rerun doesn't duplicate relationships

Since organisations are only modified on import, this seems like a
reasonable alternative to adding validation to prevent duplicate
relationships from being created.