Introduce registrationStart and registrationEnd audience checks

This patch allows QuickSurveys to target users based on registration
date. Users can define two new keys :

• registrationStart
• registrationEnd

in audiences config. Those keys have to be dates in YYYY-MM-DD
format. When key is not present given check is not evaluated (eg
user can provide only registrationStart, registrationEnd or
both at the same time. The checks are inclusive.

Bug: T218304
Change-Id: I0043066d1dd0acb1671cd8bbe508b5da9062cd75