[ticket/12341] Add tests for get_username_string()

PHPBB3-12341
Merge remote-tracking branch 'Elsensee/ticket/12341' into develop-olympus

* Elsensee/ticket/12341:
  [ticket/12341] Add tests for get_username_string()
[ticket/10851] Set disallowed content to empty array if checking is disabled

The disallowed content defaults to a standard set of mimetype triggers by
default. If one doesn't want to check the attachments mimetype triggers then
we need to set the disallowed content to an empty array.

PHPBB3-10851
[ticket/12318] Revert "Pin HHVM version to 3.0.0~precise."

This reverts commit f6614f2078740675ca0228b348500991888c2140 as the install
command started failing with "Version '3.0.0~precise' for 'hhvm' was not found"
in the meantime.

PHPBB3-12318
Merge remote-tracking branch 'bantu/ticket/12318' into develop-olympus

* bantu/ticket/12318:
  [ticket/12318] Revert "Pin HHVM version to 3.0.0~precise."
[ticket/12353] Fix displayed attachments in ACP

PHPBB3-12353
Merge remote-tracking branch 'Elsensee/ticket/12353' into develop-olympus

* Elsensee/ticket/12353:
  [ticket/12353] Fix displayed attachments in ACP
Merge remote-tracking branch 'Marc/ticket/10851' into develop-olympus

* Marc/ticket/10851:
  [ticket/10851] Set disallowed content to empty array if checking is disabled
[ticket/12359] Remove inline width styling on UCP birthday fields

PHPBB3-12359
Merge remote-tracking branch 'VSEphpbb/ticket/12359' into develop-olympus

* VSEphpbb/ticket/12359:
  [ticket/12359] Remove inline width styling on UCP birthday fields