Syntax highlighting for code blocks

This improves the MantisCoreFormatting plugin by adding syntax
highlighting for code blocks.

This implementation is based on Prism.js, using a single js file, to
make it easier to replace it with an other solution if needed in the
future.

Features:

• activate or deactivate syntax highlighting completely
• select a theme from those provided by the Prism.js main repository
• activate one or more plugins (see below)
• autoload languages as needed (i.e. ```xxxx will load language xxxx);
  with $g_cdn_enabled = ON; the resources are loaded from 1
• nothing is loaded if there are no <code> blocks

Bundled plugins:

• copy-to-clipboard: https://prismjs.com/plugins/copy-to-clipboard/
• show-language: https://prismjs.com/plugins/show-language/
• show-invisibles: https://prismjs.com/plugins/show-invisibles/
• line-numbers: https://prismjs.com/plugins/line-numbers/
• normalize-whitespace (default): https://prismjs.com/plugins/normalize-whitespace/
• match-braces (with "rainbow braces"): https://prismjs.com/plugins/match-braces/
• diff-highlight: https://prismjs.com/plugins/diff-highlight/
• inline-color: https://prismjs.com/plugins/inline-color/
• previewers: https://prismjs.com/plugins/previewers/

Fixes 0034124, PR https://github.com/mantisbt/mantisbt/pull/1986

Signed-off-by: Damien Regad dregad@mantisbt.org