Environment/queue support

Several things:

• [ ] beat.cmdline#21 (closed): Support fetching env/queue info from beat.web
• [ ] beat.web#495 (closed): expose env details (pkgs & versions) from env api
• move the string inputs to selects (select a env/version/queue instead of manually entering the name)
• per-block env/queue overrides
• integrate with samuel's pkg changes