Two small changes to the API

[Dominic Mitchell]

This is advanced warning that two small changes will be introduced to the DOAJ API shortly:

1) OutgoingAPIJournal shares 'es_type' which is redundant since it is always 'journal'. 

2) OutgoingAPIJournal shares 'admin.in_doaj' but only published journals (journals in DOAJ) are available via the API, so this is always 'True'.

Both these will no longer be shared because they are always the same value.

(OutgoingAPIJournal is our internal name in code for what we refer to in docs as the Journal retrieve endpoint. The journal search endpoint isn't affected (in_doaj and es_type are already absent from there)

Let me know if you have any questions.