Add `FLUENT_PAGES_DEFAULT_IN_NAVIGATION` setting

Allows larger sites to set the default "Show in navigation" choice.
(cherry picked from commit d2557485656d2f3f8981be9db367e4387bd0a5d7)

Conflicts:
	CHANGES.rst
	fluent_pages/appsettings.py
Widen modification_date column, to support other languages.

E.g. Italian output: "mercoledì 11 settembre 2013 00:16:34"
(cherry picked from commit a9652383c4715282ebee353a2a87a50b17331d45)

Conflicts:
	fluent_pages/static/fluent_pages/admin/pagetree.css
Also add url_pattern_types() to the manager class.

(cherry picked from commit 38b25d76f95b81bc95b9206921ab5445ba6e800d)
Fix using `{% appurl %}` for modules with multiple results.

(cherry picked from commit 556e5f98a17b11dd50de34927856c06366ec22ee)
Add notice about _current_fluent_page.

Turns out django-fluent-blogs code actually reads this variable too.
(cherry picked from commit de6a82ef010e943e54150fa0e3493a350939b32d)
Add traceback to runtests.py

(cherry picked from commit 8707de18b5176fe94ec573ccbf7264014148c26e)
admin: warn when fluent_layouts.ct_id is not defined in the template.

(cherry picked from commit ab28197a23bbf350df5bde5118e1b12d437d1220)
Update version and changelog to 0.8.6