Print warning when error ignored due to $g_display_error settings

When $g_display_error[E_USER_ERROR] != 'halt', MantisBT application
errors are ignored. This may lead to data integrity issues and may cause
the system to function incorrectly.

We now display a warning message after the inline error, so that the
user is aware of the situation

Fixes #12632
Print warning on login page when debug settings are in use

Some config settings are intended for development or debugging purposes,
and should normally not be used on production systems:

- $g_show_detailed_errors != OFF
- $g_display_errors[E_USER_ERROR] != 'halt'
- $g_debug_email !==  OFF

We now check for these cases and display a warning on the login page
when these are set incorrectly.

Fixes #16059
Remove obsolete comment
Admin Checks should warn when debug settings are used

This implements the same logic as on the login page.

Fixes #16059, #12632
Fix #16025: Change DOCTYPE to HTML5 on Admin Checks page
Admin check page: fix incorrect HTML
Documentation: remove reference to obsolete config

$g_show_queries_list was made obsolete in commit
3ea2997ae1b2c70055dd276dc15ea8f6337ca50b
Force reporting of E_USER_* in error api

Solves the problem of Mantis silently proceeding through errors that
should effectively stop code execution, when error_reporting is disabled
in php.ini or apache mod config.

This follows @davidhicks recommendation in comment ~22998.

Fixes #10966
Fix #16061: Added LOG_ALL constant

This enables full use of bitwise operators to set the log channels
($g_log_level) in config_inc.php.
Change example developer setting for $g_display_errors
Documentation: added Debugging configs

Includes updated phpdoc comments in config_defaults_inc.php