Fix two small code error in examples.

Save form_overrides into CustomModelConverter, to fix #421, in conjunction with https://github.com/coleifer/wtf-peewee/pull/20

Inline model field fix to address issues #417, #418

Missing "op" alias for os.path

Use `send_file` instead of `send_from_directory` to serve files in FileAdmin

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.

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.

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.

Do not enforce github ribbon over http

Docs in readthedocs run over https and Chrome recognize the ribbon as unsafe content.

Inline view fix

Remove inline_view from contrib/peewee/form.py, so as to fix #406 (cleaned up this time)

This reverts commit c424c3fe.

This reverts commit 749161e7.