Proposal: rename app.id to app.post

[cuibonobo]

Currently the post schema lists app.id as the identifier of the post that contains more information about the app. However, other post members that store similar information are named with .post instead. For example, permissions.groups[].post, version.parents[].post, mentions[].post, etc.

I propose changing app.id to app.post in order to maintain a more consistent naming convention within the post schema.