[ticket/12969] Add template events around username link on composing pm

Add template event before/after find username link on composing pm
to allow extensions perform additional actions on user finding.

PHPBB3-12969
Merge pull request #2868 from rxu/ticket/12969

[ticket/12969] Add template events around username link on composing pm
Merge branch 'develop-ascraeus' into develop

Conflicts:
	phpBB/styles/subsilver2/template/ucp_header.html