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.
The beta build currently has a couple of UI elements (the horizontal
logo and the app header) set to "dark mode", even though nothing else
in our UI supports dark mode yet. These styles have been removed. We
should make the entire UI support dark mode all at once at a later
date.
Also the horizontal logo used for beta was named incorrectly; the dark
and non-dark logos were swapped. They have been switched back so that
the logo that ends with `-dark` is for dark mode.
