Settings: remove obsolete ADMIN_MEDIA_PREFIX varible
Settings: in debug mode, let Django serve static files from STATIC_ROOT

With Django 1.4 the staticfiles application got included and the
variable names changed. STATICFILES_LOCAL isn't used anymore and got
replaced by STATIC_ROOT. This commit makes CATMAID's URLs config
respect that.
Settings: use MEDIA_ROOT and MEDIA_URL as storage locations

The settings files should not contain any traces to static and media
file configurations that refer to Django < 1.4 settings (like
STATICFILES_LOCAL and STATICFILES_URL).
Settings: move debug settings up a bit
Settings: add a top comment for consistency
Settings: use subdirs of MEDIA_ROOT for output in HDF5 and cropping tools
Settings: make settings generator Python 3 compatible
Settings: let the settings generator replace douple-slashes

Double-slashes will occur in the generated settings files when CATMAID
lives not in a sub-directery, but the domain's root.
Settings: use correct import URL placeholders in settings templates
Settings: let the settings generator create a secret key for Django
Settings: update the setting generator template file
Settings: add leading/trailing slash checks to settings generator
Settings: rename settings_apache.py to settings_production.py
Settings: remove not needed STATICFILES_DIRS variable

This variable is only needed to define additional directories from
which static files should be collectod from. CATMAID currently works
well with the default directories.