Refactor main.js content

This merge request re-organizes some things in order to prepare the way for a more thorough refactoring of the main.js script with the following goals:

• organize main.js actions for better maintainability
• reduce code footprint that is required on all pages

For now, all this does is:

• removes dispatcher.js and moves each of its methods into a more appropriate location (shortcuts setup, GFM autocomplete both go into ./behaviors and the rest all go directly into main.js.
• moves all of the shortcuts_*.js files into ./behaviors/shortcuts
• moves findAndFollowLink into ./lib/utils
• moves svg4everybody into ./polyfills and reorganizes existing polyfills

I plan to refactor the Shortcuts class and all of its sub-classes in a follow-up MR such that we do not need to maintain a black-list of pages with which to instantiate it.