Make Session's `group`s use Group presenter

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 In addition, `session` still uses outdated logic that does not return activity-page URLs for open groups.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!