[ticket/12922] Posts per page in MCP should have a min value of zero

In order to allow displaying the whole topic in MCP=>Main=>View topic,
the min value in the html input field must be 0.

PHPBB3-12922
[ticket/12922] Subsilver2 fix

PHPBB3-12922
[ticket/12903] Remove dead phpbb\extension\metadata_manager::_validate_version

PHPBB3-12903
Merge pull request #2821 from bantu/ticket/12903

[ticket/12903] Remove dead phpbb\extension\metadata_manager::_validate_version
Merge pull request #2816 from n-aleha/ticket/12922

[ticket/12922] Posts per page in MCP should have a min value of zero

* n-aleha/ticket/12922:
  [ticket/12922] Subsilver2 fix
  [ticket/12922] Posts per page in MCP should have a min value of zero
[ticket/12853] Hide moderator link if it is really necessary

PHPBB3-12853
Merge pull request #2829 from nickvergessen/ticket/12853

[ticket/12853] Hide moderator link if it is really necessary