Add static caching to project_release_get_api_taxonomy() and project_release_get_release_type_vocabulary()

For some reason, I was under the mistaken impression that taxonomy_get_tree() cached its results. It does not. We call project_release_get_api_taxonomy() a lot, and it's never going to change during a page load, so we should just stash the $tree in a static and reuse it once we've computed it once. Ditto project_release_get_release_type_vocabulary().