admin: Add groups in user details

Fix apyrgio/synnefo#161
admin: Format dates properly in list views

Use the `strftime` method of `datetime` type to format the dates like
this:

    Year-Month-Day Hour:Minutes

Fix apyrgio/synnefo#162
admin: Show user email in details href

Fix apyrgio/synnefo#163
admin: Add associated items in IP history summary

Fix apyrgio/synnefo#164
Merge branch 'feature-admin-last-minute' into feature-admin