use proper checks in update_meta_value etc

[tharsheblows]

Currently when you update a meta value, it checks current_user_can('edit_post_meta') regardless of the actual object type. It needs to use object type dependent capability checks. These aren't in core yet but there's a ticket here:
https://core.trac.wordpress.org/ticket/38284

A workaround for the time being would be to check edit_{$object_type} I suppose.

[tharsheblows]

When I looked at it a bit more in depth, I ended up with this explanation of the core issue: https://core.trac.wordpress.org/ticket/38303

I think I was a little unclear before -- what I meant to say was that comment meta, term meta and user meta can't be updated using the REST API due to the issue in core.

This is what the fix looks like:
tharsheblows@e71d4bd

edit: no, if need be, filters can be added to add_metadata etc in lieu of the current_user_can checks I think.