[ES] Adopt new style guideline for capitalization in button text: "Sentence case"

[kdmukai]

We are inconsistent in English, but going forward button text should use "sentence case" where the capitalization is the same as you'd use for a sentence. So in this case we'd want to change the buttons:

New capitalization:

• "Verificar cambio de multifirma"
• "Saltar verificación"

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Note that TopNav titles should be "title case": Every major word is capitalized (actually, when in doubt -- or maybe always? -- capitalize even the minor words, too: e.g. "A" in the example below).

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Note that there are exceptions for words or phrases that we treat as proper nouns:

"SeedQR" should always be capitalized.

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See matching issue for base English strings: SeedSigner/seedsigner#648