Expose the Icon Theme through the REST API

ATM is not possible to use the Icon Theme from JavaScript code unless:

• you mix the JavaScript code with Velocity
• or you read the icons from HTML (where they were placed by some server side script)
• or you get the icons from some service (e.g. search suggest)

Ideally, a pure JavaScript module should be able to request from the server side the icons it needs, in the same way it asks for the translation keys (XWIKI-18491). For this we should probably add a new REST resource that could look like this:

/xwiki/rest/wikis/{wikiName}/iconThemes/{iconTheme}/icons?icon=one&icon=two&...
/xwiki/rest/wikis/{wikiName}/iconThemes/silk/icons?icon=one&icon=two&...

or

/xwiki/rest/wikis/{wikiName}/iconSets/{iconSet}/icons?icon=one&icon=two&...
/xwiki/rest/wikis/{wikiName}/iconSets/fa/icons?icon=one&icon=two&...

and return a list of icon meta data (XWIKI-15389). I'm not sure how to ask for icons from the current icon set/theme though. Maybe by omitting the icon set/theme name.