Demo: Experimental org api

[R2ZER0]

Adds an experimental API to access info about organisations:

• /experimental/org will give a list of known Org IDs - Note they aren't unique, any may be duplicated between the Funder/Recipient/Publisher tables and so the same Org ID appear multiple times.
• /experimental/org/<org_id> shows detail about a given Org ID, split into funder/recipient/publisher subobjects

Todo / Open Questions:

• Can an organisation appears multiple times within each Funder/Recipient/Publisher tables, and so we should be returning lists? e.g. orgs with multiple IDs?
• Make the List View unique, don't output duplicate identical OrgIDs
• Figure out exactly what structure we want for the Detail View

I've removed the changes used to make my local setup work, so this only contains API changes.

TODO

• [x] Black-format the db migration file
• [x] Fix the tests

[michaelwood]

Note to selves - migrations here seems to be missing from live? did this get run not in a deploy - investigation needed before merge & deploy.

[R2ZER0]

I've rebased (and force-pushed) to remove the "replace unique_together with UniqueConstraint" commit and put that into it's own PR https://github.com/ThreeSixtyGiving/datastore/pull/162 so we can consider them seperately

[R2ZER0]

and another rebase/force-push of the commit which adds UniqueConstraints for Recipient/Funder, to regenerate the db migration, because the old one depended on the earlier migration which is now in seperate PR (also black format)