More DRY "My Account" menu

The "My Account" section menu items were repeated in the megamenu template
(``megamenu.html``) and the base template for the "My Account" section
(``storybase_user/account_base.html``)

I wanted a solution for rendering menus that:

* Allowed for identifying the currently active item
* Allowed registration of views from 3rd-party apps
* Could render the same menu with different surrounding HTML markup

I think I came up with a pretty DRY solution.  I added a new module
``storybase.menu`` that defines a ``MenuRegistry`` and ``Menu`` class.
A single ``MenuRegistry`` instance is instantiated and available as
``storybase.menu.registry``.

I create a ``Menu`` instance, add the url names corresponding to my views,
and register the menu in ``storybase_user.views``.

Finally, I created a new template tag library ``storybase_menu`` with
a template tag ``storybase_menu`` which renders a specified template with
a context that contains the menu items.

I broke out the markup for the two different instances of the menu into
separate template files: ``megamenu_account_menu.html`` and
``storybase_user/account_menu.html``.

I still need to document the ``storybase.menu`` module and the
``storybase_menu`` tag library.

Addresses #544.
Document menus feature

Document the classes and template tags used to implement the new
"My Account" menus.

Addresses #544.
Merge branch 'develop' into 544-my_acct_menu_order
Highlight the currently active item

Highlight the currently active item in the "My Account" sidebar menu.

This is achieved by giving the ``ul`` element the same ``sibiling-nav``
class as used for the Django CMS generated menus.

Closes #544
Move "View All Stories" button

On project and organization detail pages, move the "View All Stories"
button higher up on the page, below the item summary/desription.

I also added a ``view-all-stories`` class to the button (actually it's
an ``a`` element with the ``button`` class) to make the element easier
to select. I then added a Google Analytics event to track clicks for
this.

Finally, I removed some unused jQuery plugins from the ``extrajs`` block
of the ``storybase_user/detail_base.html`` template.

Closes #531
Update changelog
Add featured story count

Add the featured story count to the links at the end of a story that is
featured in the homepage slider.

Addresses #629
Show link to connected stories in explorer

Show a link to the connected stories in the explorer.  In order to do
this, I added a read-only ``connected_count`` field to the StoryApi
response. This is generated by the ``Story.connected_count`` utility
method.

As the number of stories with connected stories is somewhat small, there
doesn't seem to be any noticeable performance penalty for the extra
database call to get the number of connected stories. If this changes,
we could cache the value and invalidate using a signal, similar to how
other ``ManyToMany`` fields on the story model are handled.

Closes #629
Update changelog to reflect #629
Fix broken test

I forgot to update ``storybase_story.tests.StoryResourceTest.test_get_story``
to reflect the ``connected_count`` field added in e947ef765bccc1bc622dd9b67215d9ca74e39699

Addresses #629