Last 100 entries improvements: better sortable/searchable user list for admin functions

[lbdsgn]

Currently, to find members for the purpose of editing or adding to Challenge/League or to a Group, one must search individually, then edit individually.
The only list view shows 100 recent weight entries, which is an incomplete user listing.

On a busy site, new users would not be in the top 100 recent weight entries, limiting the admin utility of this listing.

1. If the list showed all users, AND if we could sort that list by date account created, as well as the existing last activity, then we'd have an easier time adding new students to a Group, assuming the use-case of a course beginning, so a bulk addition of new students who should be in the same Group.

2. Then a bulk-select to bulk-add, as you can do with pages, posts, and plugins, would be life-saving.