new port: www/rubygem-cal-heatmap-rails

Cal-Heatmap is a JavaScript module to create calendar heatmaps
to visualize time series data. This gem packages it for the
Rails asset pipeline.

WWW: https://github.com/pavolzbell/cal-heatmap-rails

PR: 199784
Differential Revision: https://reviews.freebsd.org/D2499
Submitted by: Torsten Zuehlsdorff <ports@toco-domains.de>
Approved by: mat (mentor)