add aria attributes to cake day elements

[e-five256]

adds aria-hidden and aria-label attributes to the new cake day locations

• cake icon on user page sidebar is decorative, as the label is already in the row, so gains aria-hidden=true
• cake icon on user popover is meant to denote that's is the user's cake day, so gains an aria-label=Cake day element surrounding it
• switched the padding to match how it is for other divs icons (if you compare with say follower count). we seem to use padding right .5rem on div i and then margin-left .5rem on btn spans (like the icons in the purge buttons)... I wonder if we should unite these, or it doesn't really matter

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