Fix Rails 6.1 deprecation on class method scopes
Fix Rails 6.1 deprecation on class method scopes

Fix the following deprecation warning:

    Class level methods will no longer inherit scoping from `inactive` in
    Rails 6.1. To continue using the scoped relation, pass it into the block
    directly. To instead access the full set of models, as Rails 6.1 will,
    use `Feed.unscoped`, or `Feed.default_scoped` if a model has default
    scopes.

The archived scope here does not need to inherit any existing scopes or
conditions from Feed. It is therefore changed into
`Feed.unscoped.archived`.

openmensa/openmensa,openmensa/openmensa,openmensa/openmensa,openmensa/openmensa