Prototype of API explorer, refs T168

Summary:
  # Made a rudimentary layout
  # Added a custom JSON syntax highlighter using `JSON.stringify` to pretty print it
  # Feature projected both the view and the link to api explorer in the tools menu
  # Implemented most of the endpoints

Still not sure how to handle POST, PUT and DELETE methods in terms of how to pass the data. Should we generate sample data for submission or allow users to submit their own sample data. Do we want to allow data manipulation via the API explorer ?

Test Plan:
- regenerate `style.css`
- go to `#settings/sites`
- enable `api_explorer`
- go to `#apiexplorer`
- click some methods

Reviewers: shadowhand, rjmackay

Reviewed By: shadowhand, rjmackay

Subscribers: sebm90, eyedol, seth

Maniphest Tasks: T168

Differential Revision: https://phabricator.ushahidi.com/D17