All "current doc" checks should run through Docs wrappers

[boonebgorges]

I ran into a serious problem on a client site because of the use of get_the_ID() on the edit page. The client was using buddypress-docs-in-group, which causes the Doc to appear in a different place in the WP page hierarchy (Groups rather than the single CPT). For some reason, get_the_ID() was properly filtered by our custom plugin prior to Docs 2.0, but not afterward, which caused some enormous headaches.

I haven't dug up the root cause for the 2.0 change, but we should be able to mitigate future damage by always using bp_docs_get_current_doc() and other filtered functions whenever referencing the current Doc. No direct uses of get_the_ID() (except in loops) or get_queried_object_id(). That way, the values can be reliably filtered.

[boonebgorges]

I've made enough changes here to avoid the specific problems that my client experienced. I'm feeling too conservative to make more changes, so I'm going to call this done.