Allow symlinks for the themes directory too

[silviot]

We should mount individual targets of symlinks if any are present in the themes dir of a project, in a similar way as what we do wrt requirements in

derex.runner/derex/runner/project.py

Lines 247 to 252 in 3b0cf4d

	for el in self.requirements_dir.iterdir():
	if el.is_symlink():
	self.requirements_volumes = requirements_volumes
	requirements_volumes[str(el.resolve())] = (
	"/openedx/derex.requirements/" + el.name
	)