Create more consistency throughout Calypso by standardizing sentence case on button labels.
Why sentence case?
Sentence case improves readability by allowing users to see proper shapes of words without the breaking flow of Capital Letters.
Sentence case respects the difference between proper nouns and the other words.
i18n: some languages capitalize different things (e.g. all nouns are capitalized in German). Title case adds cognitive load when users have to figure out which words are nouns, for example.
