Only load status_config.php CSS when necessary

The dynamic CSS for status colors was included in relationship_api.php
and bug_group_action_api.php, which caused status_config.php to be
loaded for every MantisBT page even though it is not needed for many of
them.

This moves the require_css() call to the individual files that require
the specific color classes.

NOTE: This could introduce regression issues for Plugins relying on the
status_config.php CSS to be loaded by MantisBT core.

• Affected plugins: relying on html_get_statuscss* functions
• Symptoms: The status indicators are displayed in black instead of the
  expected color
• Solution: add require_css( 'status_config.php' ); to the relevant
  plugin pages or in the EVENT_LAYOUT_RESOURCES hook, as appropriate.

Fixes 0030773