make max image number configurable

[mashb1t]

See #1248
See #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 #1013.

[siriume]

I think it might be better to add it to presets at the same time, lots of them don't know how to modify it.