In discussion with nanaya, we likely want this now that we're adding the
ability for some icons to be extended. Historically mod icons have been
displayed in a stable but arbitrary order (based on their position in
the `Mods` enum).
This change aims to make them stable across lazer scores (where they are
stored based on .. the order the user selected them).
The old flags used a size of 150x100, while the new assets based on
twemoji use a size of 150x108. Update existing usages to match this new
aspect ratio better.
Previously such use cases have been returning `null` values just fine since `GetLocalisableString` was returning `LocalisableFormattableString`, and these null values were wrapped in `LocalisableString`s implicitly, therefore not requiring an explicit fallback string and `null` doesn't break anything.