Upgrading django-oauth-toolkit to 1.5 and above

[stuart-bradley]

In django-oauth-toolkit 1.5, there is a change to the AbstractGrant.redirect_uri which changes the field from a CharField to a TextField (link). This means that to support both 1.3 and 1.5+ we need to support two different migration paths in drf-integrations-framework.

Ideally, we should just have a hard cut-off at 1.5 to resolve the issue, but this we are currently supporting services using 1.3.2.

[stuart-bradley]

When jumping to django-oauth-toolkit@2.* there are a number of non-backwards compatible changes:

• Migrations that modify the abstract models.
• Introduction of OAUTH2_PROVIDER_ID_TOKEN_MODEL and oauth_provider.IdModel.
• Introduction of oauth2_provider.models.ClientSecretField
  Since non of this is in < 2.*, it is not likely possible to upgrade and maintain backwards compatibility between versions.

[stuart-bradley]

Close via #50