File upload change

Breaking change for file uploads.
Instead of a data dict containing all formdata,
a files dict is in the following endpoints

• emoji
  • create_custom_emoji() takes emoji_name additionally to a files dict
• files
  • upload_file() takes channel_id additionally to a files dict
• brand
  • upload_brand_image()
• saml
  • upload_idp_certificate()
  • upload_public_certificate()
  • upload_private_key()
• system
  • upload_license_file()
• users
  • set_user_profile_image()

See the documentation for an example.

Thanks to @ablepharus for his PR! :-)