Refactor views for share and embed popups

Refactor the views that generate content for the popups shown
when the user clicks the "Share" or "Embed" buttons on Story,
Project and Organization detail pages to limit repeated code
and disambiguate names.

Rename the views driving the share and embed button popups to avoid
ambiguity with the view that handles content for the widget used to
embed stories on other sites, ``StoryWidgetView``. For example:

StoryEmbedWidgetView -> StoryEmbedPopupView
StoryShareWidgetView -> StorySharePopupView

These are subclasses of the base views
``storybase.views.sharing.EmbedPopupView`` and
``storybase.views.sharing.SharePopupView`` that use the templates
``storybase/embed_popup.html`` and ``storybase/share_popup.html``.
These should be used as the base views for all embed and share
popups, and subclasses need only specify the ``queryset``
attribute.

Instead of having separate template tags for getting the embed
codes of items: ``story_embed``, ``organization_embed`` and
``project_embed``, there is an ``embed_code`` template tag.

Modify CSS class names so they use ``storybase-embed-button`` and
``storybase-share-button`` instead of ``storybase-*-widget``.
Use ``embed-popup`` and ``share-popup`` as the container classes
instead of ``share-widget``.

Addresses #824