Make themes npm installable

[julianlam]

The final step in the theme manager integration is making them installable via npm.

Before this issue can be considered complete, the following must be done:

• NodeBB must be able to detect installed themes by scanning the node_modules directory (much like plugins do, at the moment)
• A theme must be published with a theme.json file
• The install script must set the vanilla theme if no theme is configured
• NodeBB must load the theme's LESS into the header via less-middleware
  • Note: Because less-middleware was used only for CSS, you can possibly just change the existing code to point to the node_module folder. Don't forget an express.static call.
• Vanilla, Cerulean, and Modern must be moved out of public/themes and into their own npm packages
• public/themes is to be deleted
• The wiki page must be updated to comply with this new information