Group code custom validity: no need for placeholder

[smadbe]

Overview

On group composition page > group code > custom validity time:

That's not really clear what these "DD" are for.. especially when "D" is not the translation for day in the language ;-)... especially as the labels on top of the field are very clear.

Acceptance criteria

• remove the DDD, HH, MM placeholder from these input fields.
• rename "Validity" label to "Lifetime"