Tweak the Fluent string letterboxing-size-status to be easier to translate and work better on Weblate

In letterboxing-size-status we use the word "Letterbox", but previously we consistently used the form "Letterboxing" when mentioning this feature in the UI. Moreover, we asked our translators to also use the form "Letterboxing" to describe the feature. Therefore, for consistency and to avoid having to get translators to figure out how to work with "Letterbox", I think we should switch to using "Letterboxing".

The comment added in !1469 (merged) needs a little tweak to work for Weblate: add a full stop and prevent duplication between the group comment and message comment.