Exit group context on <front> links, user links

Problem/Motivation

After fixing all the routing behavior, we have discovered a couple of places where the group context is preserved when we want to exit it.

Steps to reproduce

With group_purl set up:

1. Click on the logo in the site branding area when in a group -> you end up on the group page, not the home page.

2. Log in as a user who is a member of at least one group -> you end up in the group context - e.g. /business/users/me

... combined this makes it hard to get to the home page!

Proposed resolution

Exclude the front page route and user entities from the group context -- always exit the group context for those routes.

Remaining tasks

Create tests, and fix.

User interface changes

API changes

Data model changes