MantisGraph: new method to load chart.js resources

Since MantisBT 2.8.0, MantisGraph only loads chart.js for its own pages.

This prevents other plugins from accessing these resources to publish
their own charts, unless they bundle chart.js themselves.

This commit exposes a dedicated public method include_chartjs() to
include the library, separately from the plugin's standard resources()
function.

Fixes 0027122