Set primary_photo's [CHAR LIMIT=25]

Originally, I hoped to avoid line wrap by keeping translations
under 15 characters and expecting translators to fallback to
"primary" instead of "primary photo".

Prompting by translators have made me reconsider. The editor is confusing
enough without cutting half of descriptions off. And, the word wrap
doesn't look too bad anyway.

Bug: 18485409
Change-Id: I0847843f14e43c4246991610fe7eeff2d803b59e