REST API: Clean up argument and property types

[jnylen0]

Fixes a few remaining issues with argument and property types, found using ​this script to automatically check endpoint arguments and item schemas. (Current output of the script against trunk is ​here.)

• All args/properties should have a type and description
• All array args/properties should have an item type
• All context and capabilities properties should be consistently typed
• A couple of checks specific to our object properties with rendered and raw fields.

Follow-up to https://core.trac.wordpress.org/ticket/38531#comment:21.

Putting this in 4.7 because this is a good change for API consistency and because we currently allow some funky things like sending an array as the post password, so there are some minor BC implications here.

[joehoyle]

In 39250:

REST API: Clean up argument and property types.

There's a couple of places where we were missing type notes for arguments, which mainly affects documentation.

Props jnylen0.
Fixes #38792.

[rmccue]

@jnylen0 @joehoyle I'm wondering: should we convert this script to a unit test? Seems like a good way to ensure future arguments/endpoints are registered with all the necessary pieces pretty nicely.

(We could get the index for wp/v2 instead of the root if we're concerned about stepping on other components'/plugin's toes.)

[joehoyle]

In 39954:

REST API: Unify object access handling for simplicity.

Rather than repeating ourselves, unifying the access into a single method keeps everything tidy. While we're at it, add in additional schema handling for common parameters.

See #38792.

[ocean90]

In 39957:

REST API: Unify object access handling for simplicity.

Rather than repeating ourselves, unifying the access into a single method keeps everything tidy. While we're at it, add in additional schema handling for common parameters.

Merge of [39954] to the 4.7 branch.

See #38792.

[boonebgorges]

In 39992:

Revert to pre-4.7 behavior for fetching object instances by id.

This changeset reverts [38381], which caused inconsistencies in the way the
REST API fetches posts and other objects.

See #38792, #37738.

[boonebgorges]

In 39993:

Revert to pre-4.7 behavior for fetching object instances by id.

This changeset reverts [38381], which caused inconsistencies in the way the
REST API fetches posts and other objects.

Merge of [39992] to the 4.7 branch.

See #38792, #37738.