make max image number configurable

[mashb1t]

See https://github.com/lllyasviel/Fooocus/pull/1248 See https://github.com/lllyasviel/Fooocus/discussions/1511

Uses default_max_image_number as upper limit for default_image_number while keeping the default behaviour if not explicitely adjusted.

⚠️ Increasing default_image_number to a high value (depending on performance selection and GPU, roughly around > 5-10) may cause Gradio to lag heavily though. Therefore best used in combination with https://github.com/lllyasviel/Fooocus/pull/1013.