feincms_nav: Fix regression with level=somewhere deep in the tree

Thanks to Daniel Mettler for the report.
Support nested ContentObjectMixin processing

Convert extra_path into a list.  Nested ContentObjectMixins make this
necessary, for example elephantblog entries inside application contents
on page.
Switch from mimetype to content_type when initializing HttpResponse objects

Refs Django issue #16519.
Fix RichTextField form field generation.

All standard properties that would affect formfield were being ignored
(such as blank=True).
No need for 'position' field to be mandatory.
Also conditionally use content.position in ImageContent template.
Fix oddity in docs.
Fixed psges priority for sitemap
Fixed pages order in sitemap
Avoid explicit references to Page in PageAdmin
Allow FEINCMS_USE_PAGE_ADMIN=False to avoid registering default Page admin.
Allow feincms_nav to work with non-default Page models.

 -- unless no feincms_page in context; need to figure out best way to
    handle that. Maybe an additional argument?
Remove explicit use of page.Page in views; customise via page_model_path arg.

 -- needs docs.
Decouple PageAdminForm from The One Page model. See #241.
Docs: fix typos.
Add a note concerning FEINCMS_USE_PAGE_ADMIN
Merge branch 'site_map_fix' of git://github.com/PSyton/feincms

* 'site_map_fix' of git://github.com/PSyton/feincms:
  Fixed pages order in sitemap
  Fixed psges priority for sitemap
Merge branch 'master' into next

* master:
  Fixed pages order in sitemap
  Fixed psges priority for sitemap
  Fix oddity in docs.
  Fix RichTextField form field generation.
  feincms_nav: Fix regression with level=somewhere deep in the tree
Merge branch 'next' of git://github.com/pjdelport/feincms into next

* 'next' of git://github.com/pjdelport/feincms:
  Docs: fix typos.
except ValueError on siblings_along_path due to page pretenders