Introduce and support skinName settings parameter

[muxator]

#3441 discusses the introduction of custom named skins.

This issue proposes the first enabling step: the introduction of a new skinName parameter in settings.json that holds the name of the desired skin.

Detailed spec:

• move current src/static/custom to src/static/skins/no-skin (this will be the default empty skin)
• in the code, identify the files that point to static/custom/*.
  DONE: they seem to be:
  • src/node/hooks/express/specialpages.js
  • src/static/js/ace.js
  • src/templates/index.html
  • src/templates/javascript.html
  • src/templates/pad.html
• replace all the references to static/custom/* with src/static/skins/*.
  For HTML files the <%=settings.skinName%> ejs syntax can be used.
• perform a sanitization of the skin value: when concatenated to settings.exports.root, it has to be a proper, existing subdirectory of src/static/skins
• in case of errors, fall back to no-skin
• backwards compatibility: when running with a settings.json from Etherpad <= 1.7, which does not have the skinName parameter, the application has to fall back to no-skin and print a WARNING message in the logs, inviting the user to update its settings file.