dropbox docs missing required settings

[bukowa]

Describe the bug

Dropbox docs are missleading.

To Reproduce

1. Go to: https://django-dbbackup.readthedocs.io/en/master/storage.html#dropbox
2. Read docs.
3. Install pip install dropbox django-storages.
4. Set oauth2_access_token:

DBBACKUP_STORAGE = 'storages.backends.dropbox.DropBoxStorage'
DBBACKUP_STORAGE_OPTIONS = {
    'oauth2_access_token': DROPBOX_ACCESS_TOKEN,
}

5. Receive

dropbox.dropbox_client.BadInputException: OAuth2 access token or refresh token or app key/secret must be set

6. Read source code https://github.com/jschneier/django-storages/blob/7914409c90f04a749e517bb39635b218d908265f/storages/backends/dropbox.py#L86

7. Set:

DBBACKUP_STORAGE = 'storages.backends.dropbox.DropBoxStorage'
DBBACKUP_STORAGE_OPTIONS = {
    'oauth2_access_token': DROPBOX_ACCESS_TOKEN,
    'app_key': '...',
    'app_secret': '...',
}

8. Everything works.

[OmarLajam]

"Dropbox has recently introduced short-lived access tokens only, and does not seem to allow new apps to generate access tokens that do not expire."
[https://django-storages.readthedocs.io/en/latest/backends/dropbox.html]

django-dbbackup does not use correct settings for Dropbox.