[ticket/11163] Add ext/index.htm

PHPBB3-11163
Merge pull request #2464 from nickvergessen/ticket/11163

[ticket/11163] Add ext/index.htm

* nickvergessen/ticket/11163:
  [ticket/11163] Add ext/index.htm
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/11163] Add ext/index.htm