contrib.admin should use a variable to select base_site template

[CBWhiz]

Currently, I have two AdminSite instances. I'd like them to be customized in different ways. For this example, suppose I want a different branding for each section (say 'Warehouse Admin' and 'Content Admin') at the top of every admin page.

The current suggested way to customize the admin is to create a new admin/base_site.html file and fill in the branding and title blocks. However, in the scenario above, both AdminSite instances will use the same base_site template.

I suggest AdminSite be given a new instance level attribute called base_template_name, have all admin views pass this variable to the template context (or create an admin ContextProcessor), and change each admin template from {% extends "admin/base_site.html" %} to {% extends admin_base %}.

Another option would be just changing the subdirectory of the include (that is, allow each AdminSite to change the '/admin' default).

I suspect this may be able to be solved externally from the AdminSite and templates with a customized TemplateLoader, but i'm not sure how to best indicate that a particular TemplateLoader be used for a particular view response, as that seems to be a global list in the settings module.

[Carl Meyer]

I lean towards being able to set the template base directory per-AdminSite, rather than pointing to the base template itself, as that gives even more flexibility if you want to e.g. just customize the change page for a single model, per-AdminSite. In any case, this is a valid use case that ought to be made easier.

[CBWhiz]

As a temporary workaround, this Middleware and TemplateLoader does the job: ​https://gist.github.com/1245626.

Note that this uses the threadlocal hack everyone seems to hate. Also note that if you're using Coffin for Jinja2 support, that will break due to ​https://github.com/coffin/coffin/issues/23 until you apply a workaround like that described in ​https://github.com/coffin/coffin/pull/24