admin: Add hard-coded role support

Add RBAC to snf-admin. Currently, this works as following:

There are three roles (= astakos groups):

* admin-readonly: Users that have this role cannot perform any actions
  on items
* admin: Users that have this role can perform selected actions to items
* superadmin: Users that have this role can perform all actions to items

All users see the same item lists and item details. As far as the
actions that every role sees, they are hard-coded for now in the
respective action definitions, e.g. allowed_groups=['admin',
'superadmin'].

Fix apyrgio/synnefo#79
Merge branch 'feature-admin-roles' into feature-admin
admin: Refactor code structure

The current code structure was leading to circular imports of the files
that are responsible for each entity. To solve this, we must break each
of those files to smaller pieces.

The pieces that each of the files are broken into are:

* views: The functions that implement the views for each entity.
* actions: The functions that have to do with the actions on each
  entity.
* filters: The functions that have to do with filtering.
* utils: Auxiliary functions.

With the new structure, the users' views can import the vms' actions and
the vms' views can import the users' actions without worrying about
circular imports.
Merge branch 'feature-admin-refactor' into feature-admin
admin: Add action buttons in details

This commit fixes apyrgio/synnefo#37, but there is still work to be done
as we must also tackle the apyrgio/synnefo#87 issue.
Merge branch 'feature-admin-details-actions' into feature-admin
admin: Fix null values in summary

Add "-" when a value is null or otherwise empty.

Fix apyrgio/synnefo#58.
Merge branch 'feature-admin-remove-null' into feature-admin
admin: Simplify empty association lists

When an item-list of an association list has no items in it, we can hide
"Expand all" and "Total 0".

Fix apyrgio/synnefo#71
Merge branch 'feature-admin-expand-all' into feature-admin
admin: Show the full name of the logged-in user

Fix apyrgio/synnefo#72
Merge branch 'feature-admin-login-name' into feature-admin
admin-ui: Add "Remove Pagination" button (simple version)
admin-ui: Massive actions - approach 1 (base)

The optimisations are for now are very limited.
Merge branch 'feature-admin-ui-new' into feature-admin