Make Session's `group`s use Group presenter

[lyzadanger]

There's some technical debt from legacy behavior in the session module in which session decorates its groups dicts differently than the new Groups presenter. The net result is that groups returned by GET /api/profile, while the same actual list as of https://github.com/hypothesis/h/pull/4818 have different properties from those returned by GET /groups(see http://h.readthedocs.io/en/latest/api-reference/ if you are curious). In addition, session still uses outdated logic that does not return activity-page URLs for open groups.

The reason that this hasn't been changed is that a number of web pages—non-client—in h make use of session.profile and its groups and I wanted to be careful about making changes that could break current functionality. But now is perhaps the time!