We should only allow project names (which are specified in an
atlantis.yaml file) that don't need to be url escaped. The one
exceptional character is '/' which we allow because users like to name
their projects to match the directory they're in. We use the same rule
that Terraform uses for workspace names.
I've also changed the characters that are replaced when writing out the
plan filename to only remove invalid filename characters instead of just
allowing alphanumeric. This is the smallest amount of change required to
ensure the filename is valid.
