Add branding theme endpoints

[adamjmcgrath]

Changes

Adding:

https://auth0.com/docs/api/management/v2#!/Branding/get_default_branding_theme
https://auth0.com/docs/api/management/v2#!/Branding/get_branding_theme
https://auth0.com/docs/api/management/v2#!/Branding/delete_branding_theme
https://auth0.com/docs/api/management/v2#!/Branding/patch_branding_theme
https://auth0.com/docs/api/management/v2#!/Branding/post_branding_theme

References

fixes #475