Further improvements to admin/test_langs.php

This is a follow-up on PR https://github.com/mantisbt/mantisbt/pull/1772.

It includes a bit of code cleanup, and brings a few additional UI
enhancements (related to issue 0027383):

• add missing error messages
• improved visibility for plugin headers in table
• add a TOC for tested plugins, allowing user to scroll straight to a
  specific one
• add buttons to navigate between the core and plugin sections

It also implements 2 extra checks for language strings:

• basic syntax check (invalid tags, unexpected end tags, open/end tag
  mismatch, etc), fixing 0030447
• Use HTMLPurifier to detect if any unauthorized tags are present, which
  is partially fixes #12242

PR https://github.com/mantisbt/mantisbt/pull/1822