Views and templates for press releases and press mentions.

This change requires a database migration. It adds a slug to both
press releases and press mentions so that they can be linked to
independently (e.g. shared on social media). It also removes the
published boolean for press releases since we already have a
release date. Having a published boolean just means somebody will
at some point forget to make a press release public around the
release date and get angry/stressed because it wasn't released on
time.

Currently navigation happens mostly through the sidebar. This is
designed to be placed at the top of the press and show press
releases by default and recent press mentions on the side with a
link to all press mentions. That will then show recent press
releases in the sidebar with a link back to the main site.
search: add search for press releases and press mentions
press: future releases should not be accessible via slug

We limit the queryset to only press releases which have been
released, thus excluding those in the future.
pep8: fix pep8 errors
press: add pagination via a pager to press releases and mentions

Pagination is done by 15 objects. This requires us to include more
bootstrap files (pager).

An includable pager template is includes so that other pages that
do pagination can now just do {% include "includes/pager.html" %}
and it will "just work".
press: only show the content of the most recent press release

Re-styling the templates for the press releases so that they show
only the most recent press release. All other should just be titles
and release dates (with links to the actual press releases).

This is not done to the press mentions because they have a different
nature but with this change the press releases in the sidebar of the
press mention page now also show the release date (and in the
sidebar for a single press release.
press: add tests for press releases and press mentions
press: split press releases and press mentions into two separate apps

This make everything more flexible for users of the cms system.
templates: make press available from the navigation

This uses the page_url templatetag which is built into django-cms
The id of the press site should be "press".
press: plugin for most recent press releases and press mentions

This adds two plugins which can be, for example, added to the main
press page's sidebar. This required some restructuring of the other
press templates to be able to reuse the templates between the two
plugins and the list pages.
flakes: fix flakes error for unused reverse

This one cropped up after reverse was removed when vies were made
compatible with the new releases and mentions plugins.
templates: use the better page_url tag for press

Django-cms will raise a Page.DoesNotExist with the normal page_url
template tag (only in DEBUG mode), but by using our temporary fix
(this is something many people seem to want to get fixed) we can
now use it as an AsTag if we want to use it without getting a
Page.DoesNotExist.
better_page_url_tags: Return empty string, not None

If a Page.DoesNotExist exception is raised we should set the value
as an empty string, not None so that we can just use it by doing:

    {% page_url "something" as my_url %}
    <a href="{{ my_url }}">...</a>

instead of doing:

    {% page_url "something" as my_url %}
    {% if my_url %}
    <a href="{{ my_url }}">...</a>
    {% else %}
    <a href="">...</a>
    {% endif %}

The former would create the url ``/None`` which doesn't look good.
press: Allow press releases app to work alone

We shouldn't require that there be a page bound to the press mentions
application.
press: Allow press releases plugin to work alone

The CMS plugin shouldn't require that there is a page bound to the
press releases app.
press: Correct minor typo in mention detail template
Merge pull request #103 from okfn/press-releases

Press releases