Can't use project names with spaces in

For example "BSD Games" or "Workflow Prototype"; these will cause an error during pipeline resolution. The underlying problem is OSTree won't accept a refspec with a space in it, hence you end up with

  File "/builds/pr001/workflow-prototype/buildstream/buildstream/_ostree.py", line 243, in checksum
    _, checksum_ = repo.resolve_rev(ref, True)
GLib.Error: g-io-error-quark: Invalid refspec Workflow Prototype/workflow-build/00000000000000000000000000000000000000000 (0)

So we can either:

1. Explicitly say that you can't have project names with spaces in, and check for them during pipeline load
2. Translate spaces into other characters or delete them in a repeatable way, avoiding aliasing

Note that this is likely to be a more generic problem; we have no official definition for what a project name can be, it's determined by what our artifact storage system accepts (which may not always be OSTree).