[ticket/12484] Add template event ucp_agreement_terms_before/after

PHPBB3-12484
Merge pull request #2394 from VSEphpbb/ticket/12484

[ticket/12484] Add template event ucp_agreement_terms_before/after

* VSEphpbb/ticket/12484:
  [ticket/12484] Add template event ucp_agreement_terms_before/after