[ticket/12406] Fix variable description of core.viewtopic_modify_page_title

Variable page_title should be described as "Title of the viewtopic page".

PHPBB3-12406
[ticket/12418] Notice displayed for feed.php

https://tracker.phpbb.com/browse/PHPBB3-12418

language/viewtopic.php isn't loaded, so MISSING_INLINE_ATTACHMENT isn't
available.

PHPBB3-12418
Merge remote-tracking branch 'Nicofuma/ticket/12418' into develop-ascraeus

* Nicofuma/ticket/12418:
  [ticket/12418] Notice displayed for feed.php
[ticket/12199] Move deprecated functions to functions_compatibility.php

Currently 13 functions are marked as deprecated:
functions.php:
function set_var()
function request_var()
function set_config()
function set_config_count()
function tz_select()
function add_log()

functions_admin:
function cache_moderators()
function update_foes()

functions_compatibility:
function get_user_avatar()
function phpbb_hash()
function phpbb_check_hash()
function phpbb_clean_path()

functions_install:
function get_tables()

8 of them are still used in the core:
add_log              Used in many places (~198 calls)
get_tables           Used in install\install_convert::get_convert_settings
			and functions_install.php\connect_check_db
get_user_avatar      Used in phpbb\user_loader:get_avatar
phpbb_hash           Used in phpbb\db\migration\data\v30x\release_3_0_5_rc1
request_var          Used quite everywhere (~997 calls)
set_config           Used in many places (~129 calls)
set_config_count     Used in a few places (~37 calls)
set_var              Used by phpbb_http_login
			and phpbb\auth\provider\apache::autologin

PHPBB3-12199
Merge pull request #2345 from Nicofuma/ticket/12199

[ticket/12199] Move deprecated functions to functions_compatibility.php

* Nicofuma/ticket/12199:
  [ticket/12199] Move deprecated functions to functions_compatibility.php
[ticket/12097] Validate_data() should work with class method

https://tracker.phpbb.com/browse/PHPBB3-12097

PHPBB3-12097
[ticket/12097] Bug fix and adding unit test

PHPBB3-12097
Merge pull request #2325 from n-aleha/ticket/12406

[ticket/12406] Fix variable description of core.viewtopic_modify_page_ti...

* n-aleha/ticket/12406:
  [ticket/12406] Fix variable description of core.viewtopic_modify_page_title
Merge pull request #2359 from Nicofuma/ticket/12097

[ticket/12097] Validate_data() should work with class method

* Nicofuma/ticket/12097:
  [ticket/12097] Bug fix and adding unit test
  [ticket/12097] Validate_data() should work with class method