OCDS for PPPs: Update key field information and releases table

[duncandewhurst]

The validator results include the following in the key fields section:

In the PPP profile buyer and suppliers have been replaced by publicAuthority and privateParties so when PPP data is submitted we should show these in the key fields summary instead of buyer and supplier.

Similarly, the releases table includes a "Purchasers" column, for PPPs this should probably be "Public Authorities".

Once the changes in open-contracting-extensions/public-private-partnerships#142 are merged then it should be easier to determine when PPP data is being submitted (the consolidated extension will be declared).

[jpmckinney]

Will no longer be an issue once open-contracting-extensions/public-private-partnerships#236 is closed.