Move percentage api to separate mixin class

This way we have only one place which defines the API and classes do not
have to reimplement it.
Create mixin class for some standard URLS

Those are mostly required by Git status page and need to be accessed by
attribute ({% url %} can not be used as we need different kwargs every
time).
Drop not needed methods

They were used just once, it's better to replace them with {% url %}
tags.
Drop not used import