Improve admin tools

[limwa]

Original Issue: semanadeinformatica/website#58

The current admin controllers are very generic and allow creating, updating and deleting models, but reading is limited to a list of models. We should add pages for individual models where appropriate. This page could show some more information, like the number of users enrolled into an edition.

We should add additional functionality where required, like requesting a user password change or maybe even sending a custom email to a particular user.

Editing a users picture is also impossible right now.