[ticket/11331] Inform user that double dots will be stripped from avatar paths

PHPBB3-11331
Merge pull request #2555 from marc1706/ticket/11331

[ticket/11331] Inform user that double dots will be stripped from avatar paths
Merge branch 'develop-ascraeus' into develop

# By Marc Alexander
# Via Dhruv Goel (1) and Marc Alexander (1)
* develop-ascraeus:
  [ticket/11331] Inform user that double dots will be stripped from avatar paths