Docs: Work on "customising an app".

The docs contained reference on how to keep template tags working after
overriding an app, but this information is now obsolete as they've been
moved to oscar/templatetags. Thus, this info is removed.
It is replaced by similar instructions on how to keep the Django admin
around.
Fix URL for category update view in dashboard.

Detail view and update view had the same URL,
update view is now /pk/update/ as elsewhere.
Merge pull request #497 from maikhoepfel/fix/categoryurl

Fix URL for category update view in dashboard.
Remove dashboard nav functionality

Better to use the setting for now.  This fixes an issue where extensions
register a new dashboard node and it clobbers the whole nav.
Merge pull request #496 from maikhoepfel/fix/admindocs

Docs: Update 'How to customise an app'
Improve Rosetta in sandbox

* Add more languages
* Disable auth requirement
Add recipe on submitting translations
Fix a few sphinx warnings when generating docs
Merge branch 'mh/django15', remote-tracking branch 'origin' into mh/django15