Buttons: Use copy for button capitalization, not CSS

[machikoyasuda]

closes #1483

There have been app-wide changes to how buttons are capitalized or not. This PR intends to make all those changes in 1 PR (but no wording changes).

What this PR does

• Removes CSS logic for button text capitalization. The capitalization must be correct in the django.po file. The reason for this is because some buttons are capitalized, but not all. So there is no 1 style that can be applied to all buttons.
• Updates django.po file with all new capitalization in English and Spanish.

What changed

These are all capitalized

• Return Home|| Regresar a la Página Principal for 404, 400, 500 pages
• Get Started (Empezar, stays the same for Es) for Agency Index page
• Go Back || Regresar a la Página Principal for Help page

These have mixed casing

• Choose your Provider Elija su Proveedor for Index page
• Choose this Benefit Elija este Beneficio for Elig Index page
• Connect your Card for Enrollment page

What didn't need to be updated but now looks different b/c of CSS

• Check status || Verificación de estatus now has mixed casing (But will have the last word changed from status to eligibility in the copy changes to come

[thekaveman]

This PR is simpler than #1498.

We can merge this first and I can rebase on this, instead of the other way around.

This one looks ready to go now without any conflicts.