Adds rake task to download SVG icons from Tabler Github repository using config/icon_...

Adds rake task to download SVG icons from Tabler Github repository using config/icon_source.yml as source and to generate a sprite file with all the SVG icons (#23980):
* @icons:download@: downloads the default icons
* @icons:sprite@: generates the SVG sprite file for the default icons
* @icons:generate@: runs @icons:download@ and @icons:sprite@ tasks
* @icons:plugin:download@: downloads the icons for a specific plugin if the plugin provides a @icon_source.yaml@ in plugin config directory.
* @icons:plugin:sprite@: generates the SVG sprite for the plugin.
* @icons:plugin:generate@: runs @icons:plugin:download@ and @icons:plugin:sprite@ tasks

git-svn-id: https://svn.redmine.org/redmine/trunk@23105 e93f8b46-1217-0410-a6f0-8f06a7374b81