Add view of all users/groups on one page

[oskarkk]

Page on which every user and their permissions would be shown. May be read only. Respective page for viewing all groups and their permissions.

I've recently migrated from PermissionsEx, and one thing I partly miss as an owner of a small server is the ability to view everything in one place (in PeX: permissions.yml file). You can scroll through a list of users and instantly see the single permissions given to certain users, you can see the groups they're in, their UUIDs (which aren't shown at all in this web interface, but it's not the concern of this issue). Without running many commands, checking multiple files or clicking through many pages, you can quickly get a grasp of information about users - only with some scrolling when needed. It's the same with groups if you don't have many permissions in them. So, it would be nice if there were pages where you could scroll through a list of users or groups and see everything, everyone in one place. For better information density (see #100) there could be a gmail-like switch ("comfortable"/"compact") to make the rows thinner. (That's the part I could help with, as that would be only a button adding a CSS class to the whole page, and in presence of that class some elements would get different values for properties like "line-height".)