refactor(montage.py): use os.path.join instead of string concatenation
refactor(thumbnail.py): use os.path.join instead of string concatenation
refactor(uploader.py): use os.path.join instead of string concatenation
refactor(uploader.py): add support for client_secret.json file with different names in credentials folder
docs(readme.md): update instructions for openai and unsplash keys
docs(readme.md): update instructions for google oauth client id json files
docs(readme.md): add information about channel.yaml file in each channel's folder