only allow 'adminster projects' (not 'administer nodes') to delete release nodes

deleting release nodes is really crappy:

a) you're losing history
b) issues pointing to that release lose their version info
c) the tarballs aren't garbage collected properly (by design) and it throws an error about unlink()
d) once it's out, it's out. if you don't want it to be downloaded, you can unpublish, but you can never really take it back. all you can do is make a new release to supercede it.
...

sadly, in spite of numerous requests about this, people with "administer nodes" permissions on d.o continue to remove release nodes. attached patch at least takes off the delete button on the release form unless you have 'administer projects' permission, which should discourage the causual (and careless) removal of release nodes. i just want to test it a little on s.d.o before i commit it and deploy it for real.