Comment heading change to enable templating

[rica-carv]

Motivation and Context

The comments area heading is hardcoded inside the class, so no way to template it or change, for instance, from a span to a div or some other html tag.
With this change it can be fully templated via the comments_template, using somethig like $COMMENT_TEMPLATE['start'] = "{LAN=LAN_COMMENTS}: <span id='e-comment-total'>{TOTAL_COMMENTS}</span>";

Description

Retained the old code, so if there's no template, it will render as usual.

How Has This Been Tested?

Tested in my theme and with v 2.3.3

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist

• My code adheres to the e107 code style standard.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests pass.