Specific file URLs

[jbmoelker]

User story

As a content editor,
I want to be able to make uploaded files available under specific URLs,
so these files are where other software expects them
and so when I migrate an existing site to Head Start, existing file URLs can be maintained.

[jbmoelker]

#205 will most likely close the previous attempt in #147. The parts still usable from #147 for this feature are the things related to file.slug: model fields, computed title, logic to construct href etc. The thing that needs a new working implementation is catching / proxying custom file urls. I think this can be achieved by extending our new redirects middleware.