"Refreshable" UI functions, better dark mode support and an interactive styling demo

New features and enhancements

• Introduce ui.refreshable (#782 by @rodja, @falkoschindler)
• Add enable and disable methods for input elements (#560, #797 by @Help-i-am-lost, @falkoschindler)
• Introduce ui.dark_mode (#342, #783 by @smojef, @falkoschindler)
• Add min/max/step/prefix/suffix parameters to ui.number (#765, #790 by @1081, @falkoschindler)
• Switch back to Starlette's StaticFiles (#714, #723 by @clueple, @falkoschindler)
• Relax version restriction for FastAPI dependency (#805, #807 by @Viicos)

Bugfixes

• Fix ui.upload behind reverse proxy with subpath (#787 by @falkoschindler)
• Fix hidden label when text is 0 (#791 by @AndrewMHenry, @falkoschindler)

Documentation

• Add an interactive demo for classes, style and props (#795 by @phoskee, @falkoschindler)
• Improve documentation for ui.timer (@falkoschindler)
• Add a demo for creating a ui.table from a pandas dataframe (#30 by @bdshoener, @falkoschindler)