This patch adds a flow for determining the download size that would
occur for a given translation. Expected use case is for low-data modes.
* New `FetchTranslationDownloadSizeAction`
* New `SetTranslationDownloadSizeAction`
* New `CouldNotDetermineDownloadSizeError`
* New `TranslationDownloadSize` object and `translationDownloadSize` on the
session `TranslationsState`