Group administrators can now add group members

+ Simple new page to add group member by username
+ Add user form needs to be updated to hide group id
Re-organise routes

+ Document ordering rules for routes
+ Simplify app.js by introducing get.js (GET methods) and post.js (POST methods)
+ Re-order routes and standardise function naming
Document naming convention of functions serving get.js and post.js
Fix welcome message

+ Also fixed a timeout issue when a user doesn't belong to any groups
Different content for visitors vs. logged in users

+ Only present links after a user is logged in
Most listings into groups

+ Must be logged in and in a group to create a listing
+ Viewing a group will now show its listings
Alphabetical ordering for route functions

+ Fixed a minor formatting issue on the viewGroup page
Fix access behaviour for listGroup and addGroupUser