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.
Set many-to-many parent relationship from foreign key
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
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.
Prevent an organisation from being its own parent

This is just a sanity check, really, since we will import orgs and
their relationships from Whitehall and they should not be editable
anywhere else.
Merge pull request #95 from alphagov/orgs-have-multiple-parents

Allow organisations to have multiple parent organisations