Do not enforce github ribbon over http

Docs in readthedocs run over https and Chrome recognize the ribbon as unsafe content.
Make FileAdmin.base_url optional.

Add a `FileAdmin.can_download` flag to enable download of uploaded files.
If `base_url` is not defined, a `.download` route will serve uploaded files.
Use `send_file` instead of `send_from_directory` to serve files in FileAdmin

The path is already validated in `_normalize_path`. Also, `send_from_directory` abort paths with backslash, breaking FileAdmin subfolders in Windows.

With `send_file`, we have a minor issue with the download URL, it has path with backslash, but `_normalize_path` fix this before download.
Don't use column name for pk

Gets the attribute name of the model's pk column, rather than the
column's name (which can differ).

This probably needs to happen for inherited keys, too, but I have not
tested.
Missing "op" alias for os.path
Use related_name to generate inline field names. Fixes issues #417, #418.
Fixed #424. Fixed automatic class name generation
Fixed #416. More flexible menu system
Fixed #431. Fixed typo in ImageUploadField.
Apply `type_formatter` after `list_formatter`.