[Template] ucp_agreement_terms_before/after

[MattF]

Identifier: ucp_agreement_terms_before/after
Location: style/template/ucp_agreement.html (located at beginning and end of the terms of agreement text)
Explanation: To add content that is in addition to the terms of agreement text shown during registration

It will be used by phpbb/boardrules to add text during registration to the terms of agreement text, adding that registering users also acknowledge that they have read and agree to the board rules by registering.

PR: https://github.com/phpbb/phpbb/pull/2394