General improvements for the filebrowser in the Django admin

Hi,

These are patches I've written to make the filebrowser more usable in the Django admin. Feel free to cherry-pick from them, or accept the pull request as a whole. The fixes include:

* Restore the filebrowser "Browse" icon in the file fields, fix support for inlines.
* Show the filebrowser in the admin index.
* Fix regexp escaping in settings.
* Apply `xframe_options_sameorigin` in case xframe options are completely blocked.