[ticket/12400] Add viewforum event to modify topics data

The event allows extensions to use and/or modify topics list and/or
topics data before dumping out the viewforum page.
For extensions which operate with viewforum page, f.e. if the topics list
and data is needed to retrieve, add, modify additional topics info
before the output loop.

PHPBB3-12400
[ticket/12397] Fix sql_unique_index_exists doc block

db_tools::sql_unique_index_exists() searches for unique indexes but not
primary key indexes.

PHPBB3-12397
Merge pull request #2310 from n-aleha/ticket/12397

[ticket/12397] Fix sql_unique_index_exists doc block

* n-aleha/ticket/12397:
  [ticket/12397] Fix sql_unique_index_exists doc block
Merge branch 'develop-olympus' into develop-ascraeus

* develop-olympus:
  [ticket/12397] Fix sql_unique_index_exists doc block
Merge pull request #2300 from rxu/ticket/12400

[ticket/12400] Add viewforum event to modify topics data

* rxu/ticket/12400:
  [ticket/12400] Add viewforum event to modify topics data