Use promises for extension API calls

Background

See trac#7271.

What to change

Where possible use Promises instead of callbacks when using the browser.* or chrome.* APIs.

This includes the following changes:

• Update documentation files
  • docs/messaging.md
• Update mock files
  • ext/content.js
• Update UI files
  • js/desktop-options.js
    • Remove callback from getPref()
  • js/issue-reporter.js
    • Remove redundant promise wrapper
  • js/popup.js
  • js/popup.blockelement.js
  • js/popup.notifications.js
  • js/popup.utils.js
    • Remove redundant promise wrapper
    • Remove callback from
      • getPref()
      • isPageWhitelisted()
      • setPref()
      • togglePref()
    • Update calls to above functions in:
      • js/popup.js
      • js/popup.notifications.js
      • js/popup.toggle.js
  • common.js
    • Remove callback from getDocLink() and update calls in:
      • js/desktop-options.js
      • mobile-options.js
      • updates.js
  • devtools-panel.js
  • i18n.js
  • mobile-options.js
    • Remove redundant promise wrapper
• Apply adblockpluschrome updates to same release branch on eyeo/adblockplus/abpui/adblockpluschrome

Further information

mobile-options.js is only being used in Firefox Mobile