Renaming language readme file
Update phpdoc for $g_custom_strings_file
Documentation: custom strings file

The Custom Strings file is now defined in $g_custom_strings_file.

Create a new section under Customization to describe it, and refer to
the new section from various places in the documentation.
Documentation: various formatting and layout updates
Fix #14099: reverting to old language strings format
Fix error_string() following revert to old string format

Did not realize during testing of issue #14099 that the error api had
been modified as well... reverting that change too.
Fix application error 0 in manage_config_workflow_page.php

This was due to missing type cast when calling check_checked() in
print_enum_string_option_list().

Fixes #16047
Fix #16051: spacing of selected item in bracketed menus
Rename constant UNABLE_TO_DUPLICATE -> UNABLE_TO_REPRODUCE

Old constant has been moved to a new section for deprecated constants at
the end of the file.
Move ERROR_BUG_RESOLVED_ACTION_DENIED to deprecated section
bug_update: better error message for invalid resolution

The error generated when target resolution is not valid as described in
issue #15653 is not very clear.

We now print a new, improved error message ERROR_INVALID_RESOLUTION,
which includes both the target resolution and status codes.
bug_update.php: validation for change in resolution

- Reopening an issue must not cause an error (regression introduced by
  035a1302792ada9ba36ed1d729542ea629de8cca)
- Don't reopened resolution unless old status >= RESOLVED
- fix a few other cases of invalid transitions

Fixes #15653
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