The build system now supports platform-specific modifications to the
manifest for each build type. The need to customize the `id` on Firefox
motivated this change.
To support this, a new directory was made in each build type directory
for manifest changes. The images currently in this directory were moved
into an `images` subdirectory.
This new `manifest` directory can include each manifest file currently
in `app/manifest`. The `_base.json` file is assumed to exist, but the
platform manifest modifications are optional.
Static files have been added for the Flask build. This includes logos
of each size and variety that we use, and it includes the 3D model JSON
file.
Closes#12427
