refactor default severity in preRenderGroupRequirements()

Problem/Motivation

The code in StatusReport::preRenderGroupRequirements() is hard to read:

      $severity = $severities[REQUIREMENT_INFO];
      if (isset($requirement['severity'])) {
        $requirement_severity = (int) $requirement['severity'] === REQUIREMENT_OK ? REQUIREMENT_INFO : (int) $requirement['severity'];
        $severity = $severities[$requirement_severity];
      }
      elseif (defined('MAINTENANCE_MODE') && MAINTENANCE_MODE == 'install') {
        $severity = $severities[REQUIREMENT_OK];
      }

What's actually happening here is:

1. If $requirement['severity'] is set, we use that.
2. If it's not then we use either:
-- REQUIREMENT_OK if we're in install
-- REQUIREMENT_INFO if not

But the default handling is split in two parts of the code which makes it confusing.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet