Allow views to get Group from any context

Problem/Motivation

Currently, contextual filters always try to get something from the URL. With Group Sites, we sometimes want the members view (or others) to live at /members rather than /group/x/members. Provide an "argument_default" plugin that allows you to choose what context provider to get the group from.

Things to consider

1. Route access checks such as GroupRelationshipCreateAccessCheck will be unaware of this, so moving the members view to /members will break the "Add member" local action.
2. This is a first iteration of the idea, we might also want a plugin that grabs the configured context from Group Sites and put it in that module instead. Perhaps that would be a better idea. Either way, the MR contains the code for now so it isn't lost.