PR https://github.com/mantisbt/mantisbt/pull/1976

As stated by @atrol in this comment, prismjs needs to be tracked to avoid using a third party that is not maintained.

Also take in consideration my remark in PR 1976:

I also think it would be a good idea to allow use of a CDN when MantisBT is configured with $g_cdn_enabled = ON;, possibly with https://prismjs.com/plugins/autoloader, but this can be done as a separate work item.

Some ideas for future improvement (not needed as part of this PR unless you're feeling fancy and got time on your hands �)

• allow the admin to pick additional languages
• enable selected plugins (via config options), e.g. Line Numbers, Show Language, Copy to Clipboard Button

Have created a PR to show how it could work.

$g_cdn_enabled

is currently not respected, not yet.

allow the admin to pick additional languages

Autoloading works very fine so far. Allowing to pick defaults, means "packaging" of a default file. Will do it later if it seems good in general.

g_cdn_enabled

Added, works fine. The themes are reduced to the basic ones, because only the base themes are delivered by "https://cdnjs.com/libraries/prism/1.29.0"