holoviz/panel: Version 0.12.4

The 0.12.4 release fixes a number of bugs and a regression of the autoreload feature in 0.12.2 and 0.12.3. Compatibility: Fix issues with --autoreload caused by changes in Bokeh 2.4 (#2755) Enhancements: Ensure user is warned if an extension was not loaded in time on server (#2765) Allow Viewer classes to be served (#2768) Add support for rendering .ico files and pathlib objects (#2757) Bug fixes: Fixed export of vtk.js module (#2562) Fix broken HTMLTemplateFormatter on Tabulator (#2730) Fix serialization issues of Panel components on ReactiveHTML (#2743) Ensure FlexBox behaves like a layout and makes its children discoverable (#2779) Ensure Plotly plots can be updated in tabs (#2747) Fix embedding of Panel apps in Flask (#2727) Ensure Spinner widget honors bounds when created from Param object (#2740) Ensure Tabulator style does not disappear after resize event (#2770) Fix PeriodicCallback errors (#2764) Fix syncing of boolean types with URL parameters (#2758) Ensure Tabulator.download_menu applies kwargs to the filename TextInput (#2763) Ensure Tabulator does not error when no Styler is defined (#2785) Documentation: Fix MaterialUI custom component example (#2680) Fix image url in Markdown reference gallery example (#2734) Add Folium thumbnail in reference gallery (#2744