mod_admin: Easier admin css customization

Allow admin pages to include custom CSS using optional template
_html_head_admin.tpl.
mod_admin: Easier admin edit page customization

Move blocks in the admin edit page to 2 separate template files, so
that the templates can be overridden by category type. Documentation
included.

Squashed commit of the following:

commit 4a68aaa89ba4c0a02e2feef866cdfd9a4c099af2
Author: Arjan Scherpenisse <arjan@scherpenisse.net>
Date:   Sun Jul 7 11:21:07 2013 +0200

    Tweaks for admin edit page customsations

commit c033a967e1d00c231965e60d230bb1f8460b198a
Author: Arthur Clemens <arthur@visiblearea.com>
Date:   Sat Jul 6 17:11:05 2013 +0200

    Enhance documentation

    Add a note about meta to show elements on category pages but not on
    instances of that category.

commit 4af0ab38f42576287a052ffb767a8d83197e72bd
Author: Arthur Clemens <arthur@visiblearea.com>
Date:   Sat Jul 6 16:42:53 2013 +0200

    Add reference to r

    Needs reference to m.rsc[id]

commit 2bffa9d2b683edf111f684a5a26ac32184334a49
Author: Arthur Clemens <arthur@visiblearea.com>
Date:   Sat Jul 6 14:41:22 2013 +0200

    Easier admin customization

    Move blocks in the admin edit page to 2 separate template files, so
    that the templates can be overridden by category type. Documentation
    included.