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Replace Http404 exception with AdminHttp404 exception in most of the
views. Also, write a descriptive message in each exception that will be
shown in 404 page.

Fix apyrgio/synnefo#242

Fix a bug in project/utils.py that displayed resources only for project
members.

Fix apyrgio/synnefo#241

The improvements are the following:

1. Do not check if <Item>Association has a qs attribute by evaluating
it, since this will actually evaluate querysets, and for potentially
large querysets we do not want this.
2. Use `prefetch_related` to get the resources along with the project
policies, thereby reducing significantly the number of queries.
3. Use a resource cache to cache the results of Resource queries, since
they amount to almost half of the queries.

With the above improvements, the queries where reduced by 40%.

Fix apyrgio/synnefo#214
Fix apyrgio/synnefo#213

Project quotas are shown in two different places:

a) In the project summary, where they have to be displayed horizontally
and
b) in the details of a project, where they should be shown vertically.

Besides showing the project quotas properly, we must show and the
requested project application resources properly, since we currently
have no direct way to show them.

Therefore, for each project we will show either the latest state of its
quotas or the latest state of its application resources.

Fix apyrgio/synnefo#215

Fix apyrgio/synnefo#166

Fix apyrgio/synnefo#137

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.