don't automatically create release nodes for HEAD for d.o conversion?

after a discussion on IRC, a few of us concluded that it'd be best to not automatically create release nodes pointing to HEAD. maintainers may not even want to provide "releases" from HEAD. however, if they do, they should be forced to manually create the release node for it, since that requires that they select the version of core their contrib is compatible with.

this has a few benefits:

1. ALL release nodes will know what version of core they're compatible with
2. irresponsible maintainers won't automatically have HEAD releases -- so totally unmaintained cruft in contrib will be cleaned out a little
3. we can forever rid ourselves of the ambiguous "HEAD" or "cvs" identifier.

however, upon further consideration, i realized the 1 glaring problem with this otherwise brilliant move: issues in the issue queues currently pointing to the "cvs" version will no longer have a version associated with them at all. :( so, if we go this route, after the update, we're going to have around 8700 issues in the queue that no longer have a version associated with them, and about 1400 issue followups trying to move an issue in/out of "cvs" that will loose that info. :(

so, either we need a brilliant solution to this, where we somehow assign all these issues to the right thing, or make unpublished HEAD release nodes but still have those show up in the issue queue, etc, etc. probably we should just set this to "won't fix" and forget it, but it's certainly a tempting option. ;)