admin: Add more info in volume table

Add the following fields:
* Size,
* Disk template,
* Updated at
admin: Add more info in volume summary
admin: Update Volume filters

Add the following filters:
* Disk template (choice)
* Index (number)
* Source image (text)
admin: Update Volume details

Also, create "details_url" template filter that can be used by any
template and create a details url for a model instance's fields.
Merge branch 'feature-admin-volumes' into feature-admin

Fix apyrgio/synnefo#172