Merge pull request #21513 from JKingsnorth/core-2846-1-improve-start-end-date-validation

dev/core#2846 Towards validation of start and end dates. Forms.

diff --cc CRM/Contribute/Form/ContributionPage/Settings.php
Simple merge

diff --cc CRM/Event/Form/ManageEvent/EventInfo.php
Simple merge

diff --cc CRM/Event/Form/ManageEvent/Registration.php
Simple merge

diff --cc CRM/Price/Form/Field.php
Simple merge

diff --cc CRM/Utils/Date.php
Simple merge