css clean-up, breadcrumbs

[krmax44]

upstream changes for okfde/fragdenstaat_de#494

breadcrumbs are now more flexible, type-safe and better documented - simply extend views with the provided class:

froide/froide/helper/breadcrumbs.py

Lines 23 to 42 in 815abaf

	class BreadcrumbView(View):
	"""
	Your view must either provide a `breadcrumbs` attribute, or a
	`get_breadcrumbs` method.
	The `breadcrumbs` attribute is a sequence of strings or tuples,
	e.g. ["Just text", ("A link", "url-pattern-name")].
	The first element of the tuple specifies the link text, the second one
	will be REVERSED to the link, so you can provide path names.
	Using the `get_breadcrumbs` method, you can provide breadcrumbs as
	above, with the difference that urls WILL NOT BE REVERSED.
	You can also directly return a `Breadcrumbs` instance.
	Example implementation:
	class MyView(BreadcrumbView):
	def get_breadcrumbs():
	return [("Foo", "/foo/"), "Bar"]
	"""